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LIBRARY 

UNIVERSITY  OF 

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,       SAN  DIEGO 


LIFE  AND  DEATH 

HEREDITY  AND  EVOLUTION  IN  UNICELLULAR 
ORGANISMS 

LECTURES   DELIVERED 
UNDER  THE 

RICHARD  B.  WESTBROOK  FREE  LECTURESHIP  FOUNDATION 

AT  THE 

WAGNER  FREE  INSTITUTE  OF  SCIENCE 

PHILADELPHIA 


BADGER'S 
STUDIES    IN   SCIENCE 


THE  HIGHER  USEFULNESS  OF  SCIENCE. 
By  William  Emerson  Ritter. 
THE  UNITY  OF  THE  ORGANISM,  OB  THE 
ORGANISMAL  CONCEPTION  OF  LIFE.  Two 
volumes.  Illustrated.  By  William  Emer- 
son Ritter. 

THE    BEGINNINGS    OF    SCIENCE.      By 
Edward  J.  Menge. 

THE  PROBABLE  INFINITY  OF  NATURE 
AND  LIFE.  By  William  Emerson  Ritter. 
AN  ORGANISMAL  CONCEPTION  OF  CON- 
SCIOUSNESS. By  William  Emerson  Ritter, 
INSECT  BEHAVIOR.  Illustrated.  By 
Paul  G.  Howes. 

LIFE  AND  DEATH,  HEREDITY  AND  EVOLU- 
TION IN  UNICELLULAR  ORGANISMS.  Illus- 
trated. By  H.  S.  Jennings. 
THE  ORNITHOLOGY  OF  CHESTER  COUNTY, 
PENNSYLVANIA.  Illustrated.  By  Frank 
L.  Burns. 

FAMILIAR  STUDIES  OF  WILD  BIRDS.  Illus- 
trated.   By  F.  N.  Whitman. 
QUESTIONS  AND  OUTLINES  IN  GENERAL 
CHEMISTRY.    ByW.S.Haldeman. 

SANITARY  ENTOMOLOGY.  By  W.  Dwight 
Pierce. 

RICHARD  G.  BADGER,   PUBLISHER,   BOSTON 


LIFE    AND    DEATH 

HEREDITY  AND  EVOLUTION 
IN  UNICELLULAR  ORGANISMS 

BY 

H.  S.  JENNINGS 


BOSTON 
RICHARD  G.  BADGER 

THE   GORHAM   PRESS 


COPYRIGHT,  1920,  BY  H.  S.  JENNINGS 


All  Rights  Reserved 


Made  in  the  United  States  of  America 
The  Gorham  Press,  Boston,  U.  S.  A. 


PREFACE 

THE  present  work  is  not  a  book  on  Protozoology,  but  on 
Genetics,  employing  Protozoa  as  material,  and  compar- 
ing the  conditions  there  found  with  those  in  higher  organ- 
isms. 

To  select  from  a  great  mass  of  varied  material,  not  yet 
reduced  by  science  to  unity,  those  features  that  appear 
most  significant  for  certain  general  questions  is  a  task  of 
difficulty,  not  unattended  with  the  danger  of  justifying 
the  critic.  I  cannot  hope  to  entirely  escape  that  danger. 
Much  that  is  of  extreme  interest  must  be  omitted,  if  any 
clearness  of  outline  is  to  be  preserved.  A  certain  one- 
sidedness  appears  inevitable,  unless  an  encyclopedic  work 
is  attempted.  The  relatively  great  prominence  given  to 
the  infusoria  in  these  lectures  is  an  example;  the  hetero- 
geneous and  still  more  imperfectly  known  genetic  phe- 
nomena in  the  other  Protozoa  lend  themselves  less  readily  to 
a  unified  presentation.  I  can  only  hope  that  the  limita- 
tions of  the  work  aid  in  defining  certain  large  problems. 

Technical  terms  have  been  avoided.  This  is  not  alone 
because  the  lectures  were  for  an  audience  not  composed 
of  specialists.  Technical  terms,  in  spite  of  their  con- 
venience, bring  many  disadvantages,  even  in  strict  scientific 
work.  They  seem  to  give  to  phenomena  a  distinctness  and 
uniqueness  which  does  not  exist  in  nature.  They  create 
separate  entities  for  things  that  are  mere  variations  on  a 
general  theme.  Any  phenomenon  has  many-sided  relations 
to  the  others ;  to  bring  these  out  we  have  not  hesitated  even 
3 


4  Preface 

to  employ  in  different  passages  diverse  designations  for  the 
same  thing.  What  we  observe  in  the  Protozoa  are  combina- 
tions of  chemicals;  of  matter  and  energy,  with  their  char- 
acteristic activities.  Technical  terms  tend  to  set  these  apart 
•  and  render  them  unintelligible ;  what  we  need  is  to  render 
them  intelligible  by  showing  their  community  with  the  world 
of  every-day  experience. 

The  book  deals  with  heredity,  variation,  evolution,  as 
present  physiology,  not  as  past  history.  It  discusses  what 
now  occurs,  not  what  may  have  occurred  in  the  past.  Hence 
discussion  of  the  origin  of  the  conditions  now  existing  will 
hardly  be  found. 

A  cknowLedgments 

To  the  Editor  and  Publishers  of  Genetics  I  am  indebted 
for  the  use  of  the  blocks  for  Figures  12  and  23,  taken  from 
Genetics,  volume  1.  To  my  wife,  Louise  Burridge  Jenningjs, 
I  am  indebted  for  the  drawing  of  the  remainder  of  the 
figures  in  the  book. 

Johns  Hopkins  University, 
Baltimore,  Md. 


CONTENTS 

.    PAGE 

I.   GENERAL  SURVEY  OF  THE  LIFE  HISTORY  IN  THE  PROTOZOA,  WITH  " 
THE    QUESTIONS    IT    RAISES.    LIFE,    DEATH,    REPRODUCTION, 
MATING,  REJUVENESCENCE,  "POTENTIAL  IMMORTALITY"    ...       13 

II.  HEREDITY  AND  VARIATION  IN  PROTOZOA,  IN  REPRODUCTION  FROM 
A  SINGLE  PARENT.  "THE  INHERITANCE  OF  ACQUIRED  CHARAO-' 
TERS."  THE  EXISTENCE  OF  MANY  DIVERSE  STOCKS  IN  A  SINGLE 
SPECIES.  CONSTANCY  OF  THE  STOCKS.  "PURE  LINE  INHERIT- 
ANCE" AND  THE  RESULTS  OF  SELECTION.  DIFFICULTIES  FOR  THE 
THEORY  OF  EVOLUTION 38 

III.  RESULTS  OF  INTENSE  AND  LONG  CONTINUED  STUDY  OF  CHANGES 
IN  A  STOCK.    INHERITED  VARIATIONS  IN  THE  PURE  RACE.    VISIBLE 
EVOLUTION 67 

IV.  CAN  WE  EXPERIMENTALLY  CHANGE  THE  HEREDITARY  CHARAC- 
TERS?   HEREDITY  OF  ENVIRONMENTAL  EFFECTS.    HEREDITY  AND 
VARIATION  IN  BACTERIA  AND  SIMILAR  .ORGANISMS     ....      85 

V.  THE  NATURAL  HISTORY  OF  MATING.  SEX,  ITS  NATURE  AND  CON- 
SEQUENCES. SEX  IN  THE  PROTOZOA.  Is  SEX  COEXTENSIVE  WITH 
LIFE  AND  NECESSARY  TO  ITS  CONTINUANCE? 106 

VI.   WHAT  ARE  THE  RESULTS  OF  MATING?    REJUVENESCENCE  AND 

MATING.    HEREDITY  AND  VARIATION,  AND  MATING  ....     141 

VII.  How  DOES  MATING  BRING  ABOUT  BOTH  BIPARENTAL  INHERIT- 
ANCE, AND  DIVERSITY  IN  HEREDITARY  CHARACTERS?  WHAT 
EFFECT  HAS  MATING  ON  THE  STOCK  AS  A  WHOLE?  DOES  IT 
INCREASE  VARIATION?  DOES  IT  DECREASE  VARIATION?  WHAT 
Is  ITS  RELATION  TO  EVOLUTION? 170 

VIII.  COMPARISON  OF  THE  GENETIC  PHENOMENA  IN  THE  PROTOZOA  WITH 
THOSE  IN  HIGHER  ORGANISMS.  GENERAL  VIEW  OF  DEVELOP- 
MENT, MATING  AND  EVOLUTION 198 


LIST  OF  ILLUSTRATIONS 

FIOUKE  PAGE 

1.  SIMPLER  FORMS  OF  PROTISTA 15 

2.  SOME  OF  THE  MORE  COMPLEX  PROTOZOA 16 

3.  STRUCTURE  OF  ONE  OF  THE  MOST  COMPLEX  OF  THE  PROTOZOA  .  17 

4.  DIVISION  OF  AN  INFUSORIAN,  PARAMECTUM 20 

5.  DIAGRAM  OF  THE  DESCENT  OF  GENERATIONS  IN  PROTOZOA     .     .  21 

6.  CONJUGATION  OF  PARAMECTUM 22 

7.  DIAGRAM  OF  THE  PROCESS  OF  DIVISION  IN  PARAMECTUM   ...  25 

8.  DIAGRAM  OF  THE  INTERNAL  PROCESSES  IN  THE  CONJUGATION  OF 
PARAMECTUM  CAUDATUM        26 

9.  DIAGRAM  SHOWING  WHAT  OCCURS  IN  ENDOMIXIS 31 

10.  REPRODUCTION  IN  AM(EBA 39 

11.  REPRODUCTION  IN  DIFFLUGIA 40 

12.  Two  PARENTS,  WITH  THEIR  OFFSPRING,  JUST  BEFORE  SEPARATION 

IN  DIFFLUGIA  CORONA 41 

13.  REPRODUCTION  IN  STYLONTCHIA 43 

14.  INHERITANCE  FROM  MUTILATED  PARENTS,  IN  DIFFLUGIA  CORONA  45 

15.  REPRODUCTION  IN  AN  INFUSORIAN  (PARAMECTUM)  m  WHICH  THE 
ANTERIOR  END  HAS  BEEN  CUT  OFF 46 

16.  REPRODUCTION  FOR  SEVERAL  GENERATIONS  OF  A  PARAMECTUM    .  47 

17.  REPRODUCTION  IN  A  PARAMECTUM  BEARING  A  SMALL  ABNORMAL 
PROJECTION  NEAR  THE  POSTERIOR  END 47 

18.  REPRODUCTION  IN  A  DEFORMED  INDIVIDUAL  OF  PARAMECTUM       .  48 

19.  DIFFLUGIA  CORONA 51 

20a.  DIFFLUGIA  CORONA 52 

20b.  DIFFLUGIA  CORONA 53 

21.  DIFFLUGIA  CORONA;  PORTIONS  OF  FOUR  FAMILIES 55 

22.  EIGHT  DIVERSE  FAMILIES  OF  PARAMECTUM,  SHOWING  VARIATIONS  57 

23.  DIFFLUGIA  CORONA,  TO  SHOW  THE  CHARACTERS  STUDIED  IN  THE 
NEW  WORK  ON  INHERITANCE 71 

7 


8  List  of  Illustrations 

FIGUBE  PAOE 

24a.  PARTS  OF  FIVE  HEREDITARILY  DIVERSE  BRANCHES  OF  A  SINGLE 

FAMILY  IN  DIFFLUGIA  CORONA 74 

24b.  PARTS  OF  FIVE  HEREDITARILY  DIVERSE  BRANCHES  OF  A  SINGLE 

FAMILY  IN  DIFFLTJGIA  CORONA 75 

25.  THE  INFUSORIAN  STYLONYCHIA -    .  81 

26.  ARCELLA  VULGARIS 83 

27.  TRYPANOSOMA  BRUCEI 97 

28.  ORGANISMS  USED  IN  DALLINGER'S  EXPERIMENTS  ON  THE  EFFECTS 

OF  HIGH  TEMPERATURES 98 

29.  CHROMOSOMES  AND  THEIR  MATING 108 

30.  DIAGRAM  SHOWING  THE  CHIEF  PROCESSES  IN  THE  CONJUGATION  OF 
PARAMECIUM  CAUDATTJM        Ill 

31.  DIFFERENCES  BETWEEN  THE  CHROMOSOMES  OF  THE  NUCLEI  IN  THE 
Two  SEXES 115 

32.  THE  ACTIVE  AND  PASSIVE  PORTIONS  OF  CELL  AND  NUCLEUS  IN  THE 
EGG  OF  THE  WHITEFISH        117 

33.  BEGINNING  OF  CONJUGATION  IN  EPISTYLIS 121 

34.  SUCCESSIVE  STEPS  IN  THE  PROCESS  OF  CONJUGATION  IN  VORTICELLA 
NEBULIFERA 121 

85.    DIAGRAMS  OF  THE  MICRONUCLEAR  PROCESSES  IN  CONJUGATION,  IN 

PARAMECIUM  AND  VORTICELLA 122 

36.  MATING  IN  THE  MOULD,  MUCOR 123 

37.  MATING  OF  SIMILAB  CELLS,  FROM  THE  ALGA  STEPHANOSPH^ERA  .  126 

38.  CHILODON 129 

39.  CONJUGATION  OF  CHILODON 129 

40.  THE  EXCHANGE  OF  THE  HALF  NUCLEI  IN  PARAMECIUM  CAUDATUM  131 

41.  MATING  OF  Two  HALVES  OF  THE  NUCLEUS  OF  SAME  CELL    .     .  134 

42.  Two  METHODS  OF  CONJUGATION  IN  SPIROGYRA 135 

43.  PROCESS  OF  CONJUGATION  OF  Two  BRANCHES  OF  THE  SAME  PLANT.  136 
43a.  BlPARENTAL  INHERITANCE  AND  THE  PRODUCTION  OF  DIVERSITY  BY 

CONJUGATION,  IN  CHLAMYDOMONAS 156 

44.  A  FAMILY  OF  PARAMECIUM  CAUDATUM,  DESCENDED  FROM  AN  EX- 
CON  JUGANT,   AND   SHOWING  HEREDITARY  ABNORMALITIES       .        .        .  160 

45.  THE  SEPARATION  or  THE  Two  GROUPS  OF  PAIRED  CHROMOSOMES 
INTO  DIFFERENT  GERM  CELLS,  IN  THE  INSECT  NEZARA  HILARIS.     .  172 

46.  THE  CHROMOSOMES,  AND  THEIR  SEPARATION  INTO  Two  NUCLEI 
WITH  REDUCED  NUMBERS  IN  THE  PROTOZOAN  MONOCYSTIS  ROSTRATA  176 


List  of  Illustrations 


FIGURE  PAGE 

47.  REDUCTION  OF  THE  NUMBER  OF  CHROMOSOMES  AT  CONJUGATION  IN 
DIDINIUM  NASUTUM 178 

48.  CONJUGATION  AND  REDUCTION  IN  THE  NUMBER  OF  CHROMOSOMES 

IN  THE  INFUSORIAN  ANOPLOPHRTA  BRANCHIARUM 180 

49.  THE  CHROMOSOMES  AND  THE  DIVISIONS  PREPARATORY  TO  MATING 

IN  PARAMECIUM  CAUDATUM 182 

50.  REDUCTION  OF  THE  NUMBER  OF  CHROMOSOMES  BEFORE  MATING  IN 

THE  RHIZOPOD  PELOMYXA 183 

51.  DIAGRAM  TO  ILLUSTRATE  How  MENDELIAN  INHERITANCE  WOULD 
OCCUR  IN  AN  INFUSORIAN  (PARAMECIUM) 189 

52.  CONJUGANTS  AND  NoN-CONJUGANTS  FROM  A  CULTURE  COMPOSED  OF 
A  MIXTURE  OF  Two  RACES  OF  DIFFERENT  SIZE,  OF  PARAMECIUM 
AURELIA 191 

•33.    PAIRS  FROM  A  SINGLE  RACE  OF  PARAMECIUM  AURELIA,  ILLUSTRATING 

ASSORTATIVE  MATING 192 


LIFE  AND  DEATH 

HEREDITY  AND   EVOLUTION  IN  UNICELLULAR 
ORGANISMS 


LIFE  AND  DEATH 

HEREDITY  AND  EVOLUTION  IN 
UNICELLULAR  ORGANISMS 


General  Survey  of  the  Life  History  in  the  Protozoa,  with 
the  Question  it  Raises.  Life,  Death,  Reproduction,  Mating, 
Rejuvenescence,  "Potential  Immortality" 

LONG  ago,  when  Latin  was  the  common  language  of 
science,  there  was  current  a  saying  that  nature  is 
greatest  in  the  things  that  are  smallest:  Natura  maxima  in 
minimis.  Badl,  in  his  History  of  Biological  Theories,  cites 
this  as  a  maxim  which  led  to  mere  superficiality ;  to  the 
degeneration  of  biology  toward  an  amusement,  with  the 
microscope  as  its  instrument ;  to  neglect  of  the  really  great 
problems  of  life.  The  naive  literature  of  the  old  fashioned 
microscopical  societies,  with  their  talk  of  the  "golden  tube," 
and  of  the  "Oh,  my!  objects,"  passed  from  one  member  to 
another  for  the  consecutive  delectation  of  their  eyes,  lends 
some  plausibility  to  this  indictment;  even  yet  the  drama 
of  life  seen  in  a  drop  of  water  has  its  purely  aesthetic  fasci- 
nation. But  such  fascination  is  not  inevitably  inconsistent 
with  a  serious  value  for  studies  of  these  creatures ;  there 
still  exist  students  who,  in  spite  of  Radl's  sarcasm,  believe 
that  the  simpler  creatures  have  something  to  teach  us  on 
even  the  deepest  problems  of  life.  I  am  going  to  try  to 
13 


14  Life  and  Death,  Heredity  and  Evolution 

present  what  we  have  thus  far  learned  from  them  on  the 
great  questions  of  life  and  death;  of  heredity  and  evolution. 
I  hope  that  this  may  serve  as  an  introduction  and  founda- 
tion to  a  general  understanding  of  these  things ;  and  that  we 
may  even  find  that  the  simplest  organisms  have  a  distinctive 
and  important  contribution  to  make  toward  such  under- 
standing. Study  of  what  actually  occurs  even  to  its  smallest 
details  does  indeed  hamper  uncomfortably  the  sweep  of  un- 
trammeled  theory,  but  if  our  aim  is  to  attain  truths  that 
are  verifiable  rather  than  theories  that  are  magnificently 
free,  we  shall  welcome  this  result. 

Let  us  set  forth  clearly  at  the  beginning,  that  in  these 
lectures  our  interest  will  not  be  primarily  in  Protozoology, 
but  in  Genetics :  in  the  problems  of  life,  its  continuance  and 
reproduction;  and  that  we  deal  with  the  lower  organisms 
only  for  the  light  they  throw  on  these  matters.  With  many 
of  the  technical  problems  which  most  interest  Protozoologists 
we  shall  therefore  have  no  dealings;  on  the  other  hand,  the 
facts  which  we  use  will  be  such  as  do  form  constituent  parts 
of  the  structure  of  Protozoology. 

The  traditional  ground  for  hoping  that  the  Protozoa 
may  aid  greatly  in  understanding  the  foundations  of  life  and 
reproduction  is  this:  As  we  pass  from  the  complex  organ- 
isms to  the  simpler  ones,  we  must  find  that  life  retains  its 
essential  nature — for  otherwise  it  would  not  be  life — while 
stripping  off  all  merely  adventitious  details.  The  highest 
organisms  are  of  interest  to  us  because  they  show  the  heights 
to  which  life  may  rise;  the  lowest  because  they  show  the 
fundamentals  of  life  relatively  unconfused.  These  lowest 
organisms  are  commonly  said  to  consist  of  a  single  cell; 
whereas  higher  ones  consist  of  an  almost  infinite  number 
of  cells  of  diverse  kinds. 

This  point  of  view  has  been  challenged  in  recent  times, 


Central  Survey 


15 


as  well  as  in  earlier  years.  It  is  admitted  that  some  of  these 
lower  creatures  appear  to  be  extraordinarily  simple  as 
compared  with  trees,  birds  and  men:  such  are  evidently  the 
bacteria  among  plants ;  such  are  amoeba  and  its  relatives 
among  animals  (see  Figure  1).  But  in  some  of  the  Proto- 


Figure  1.  Simpler  forms  of  Protista,  a  to  g,  Diverse  kinds  of  bac- 
teria, after  Zopf,  Engelmann  and  Fischer,  h,  Amoeba  radiosa,  after 
Leidy. 

zoa  we  find  an  astonishing  complexity  of  structure  (Figure  2, 
Figure  3)  and  of  function,  so  that  some  of  the  students  of 
the  group,  like  Dobell  (1911),  maintain  that  there  is  no 
ground  for  calling  these  organisms  "simple"  or  "lower"  or 
even  "unicellular" :  that  they  are  merely  small  organisms, 
diverse  in  their  plan  of  structure  from  the  larger  animals 
and  plants.  But  it  appears  evident  that  amcebse  and  bac- 
teria are  structurally  simpler  than  vertebrates,  in  that  they 
consists  of  fewer  kinds  of  differentiated  parts;  and  this 
seems  to  me  true  even  for  the  more  complex  Protozoa,  when 
compared  with  the  Metazoa.  They  may  therefore  be 
properly  called  simpler  or  lower  organisms,  meaning  thereby 
that  they  have  fewer  differentiated  parts  than  the  higher 


16  Life  and  Death,  Heredity  and  Evolution 


Figure  2.  Some  of  the  more  complex  Protozoa.  A,  Caenomorpha 
medusula  Perty,  after  Biitschli.  B,  Didinium  capturing  prey,  after 
Balbiani.  C,  Vorticella  nebulifera  O.  F.  M.,  after  Butschli.  D,  Fol- 
liculina,  after  Mobius.  E,  Stentor  roeselii  Ehr.,  after  Stein.  F,  Lacry- 
maria  olor,  after  Verworn.  G,  Stylonychia  mytilus  Ehr.,  after  Engel- 


animals.  The  interest  of  their  study  lies  partly  in  dis- 
covering what  life  can  do  with  few  differentiated  structures. 
Moreover,  it  seems  clear  that  in  their  general  plan  of  struc- 
ture they  are  comparable  to  single  cells  of  higher  animals, 
though,  like  such  single  cells  elsewhere,  they  may  be  very 


17 


Figure  3.  Structure  of  one  of  the  most  complex  of  the  Protozoa, 
Diplodinium  ecaudatum  Fiorentini,  after  Sharp  (1914).  A,  mouth;  B, 
oral  cilia;  C,  adoral  membranelles ;  D,  oesophagus;  E,  oesophageal  re- 
tractor strands;  F,  ventral  skeletal  lamellae;  G,  entoplasm;  H,  ecto- 
plasmic  boundary  layer;  I,  cuticle;  J,  caecum;  K,  rectum;  L,  rectal 
fibers;  M,  ectoplasm;  N,  posterior  contractile  vacuole;  O,  micronucleus ; 
P,  suspensory  fibers;  Q,  macronucleus ;  R,  anterior  contractile  vacuole; 
S,  posterior  ciliary  roots;  T,  dorsal  membranellae ;  U,  operculum;  V, 
motor  mass;  W,  circumoesophageal  ring. 


complex;  they  may  properly  therefore  be  called  unicellular 
organisms. 

But  whatever  the  facts  as  to  their  simplicity  or  complex- 


18  Life  and  Death,  Heredity  and  Evolution 

ity,  they  do  show  us  life  and  the  passage  of  generations 
following  a  very  different  course  from  that  which  we  see  in 
ourselves;  a  course  which  opens  to  us  many  new  vistas. 
And  they  do  present  us  a  world  of  life  in  miniature  in  which 
structures  are  condensed  into  small  space,  in  which  activities 
are  condensed  into  brief  time.  In  a  watch-glass  on  our 
table  we  may  in  a  week  see  generations  come  and  go;  may 
observe  the  rise  of  whole  faunas  and  floras ;  their  decline  and 
replacement  by  others ;  we  may  follow  in  successive  genera- 
tions the  struggle  for  existence;  may  see  natural  selection 
at  work  and  discover  its  results.  In  a  few  days  we  may 
see  the  birth,  babyhood,  youth  and  age  of  individuals,  and 
their  replacement  by  descendants ;  we  may  study  the  inherit- 
ance of  parental  traits  by  the  new  generation,  or  the  ap- 
pearance of  new  traits;  we  may  observe  how  the  population 
changes  with  the  passage  of  ages, — and  all  while  we  wait 
for  one  of  the  changes  of  the  moon. 

This  seems  to  present  a  wonderful  opportunity  for  solving 
in  a  short  time  some  of  the  problems  at  which  men  have  been 
at  work  for  ages,  so  that  students  have  taken  up  this  work 
with  enthusiasm.  But  unhappily,  when  we  cease  to  regard 
it  as  a  mere  fascinating  spectacle,  and  desire  to  establish, 
in  the  rigid  and  detailed  manner  required  by  science,  just 
what  is  happening  in  these  teeming  populations,  and  just 
what  are  the  laws  that  the  events  follow,  we  find  the  diffi- 
culties very  great,  and  our  success  perhaps  less  rapid  than 
we  might  hope.  For  to  do  this  we  have  to  come  to  know 
these  creatures  individually;  we  have  to  work  with  them  as 
we  would  with  guinea  pigs  or  with  calves.  The  distance 
of  size  between  them  and  ourselves  is  almost  as  difficult  to 
overpass  as  are  the  spatial  distances  between  us  and  the 
stars,  so  that  studying  them  individually  is  a  little  like  try- 
ing to  get  acquainted  with  the  inhabitants  of  Mars.  To 


General  Survey  19 

become  personally  intimate  with  particular  amoebae  or  in- 
fusoria ;  to  control  their  goings  out  and  comings  in ;  their 
diet  and  personal  habits;  to  interfere  with  their  social  and 
domestic  relations ;  to  feed  them  and  mate  them ;  to  make 
them  do  and  live  as  we  want  them  to  live, — this  is  what 
we  have  to  do  if  we  are  to  really  understand  their  lives, 
their  behavior,  their  growth,  their  matings,  their  heredity, 
their  evolution. 

Some  years  ago  I  devoted  myself  to  obtaining  an  intimate 
personal  acquaintance  with  the  life  and  behavior  of  indi- 
viduals among  these  creatures ;  to  study  of  their  individual 
biography  and  perhaps  psychology.  I  had  the  honor  to 
present  this  to  the  public  in  a  book  on  the  Behavior  of 
Lower  Organisms.  Building  on  the  experience  thus  gained, 
I  have  since  devoted  myself  to  what  happens  in  the  passage 
of  generations  in  these  creatures ;  to  a  study  of  the  biology 
of  races  rather  than  of  individuals;  to  life,  death,  mating, 
generation,  heredity,  variation  and  evolution  in  the  Protista. 
I  am  going  to  attempt  to  present  a  picture  of  these  matters 
so  far  as  our  present  knowledge  makes  possible.  We  shall 
find  that  there  are  still  many  questions  which  are  not  yet 
answered,  but  unsolved  problems  after  all  have  their  fascina- 
tion ;  and  much  has  been  learned ;  this  I  shall  try  to  present 
and  compare  with  what  is  known  for  higher  organisms. 

When  we  follow  the  lives  of  particular  individuals  in  the 
miniature  jungle  which  a  protozoan  culture  presents,  we 
come  upon  a  fact  that  is  most  astonishing  to  one  who  knows 
only  the  higher  organisms.  The  creatures  seem  never  to 
die,  save  by  accident.  If  we  follow  a  single  individual,  we 
find  that  after  a  time  he  divides  into  two  (Figure  4). 
Which  is  the  parent,  which  the  offspring?  Each  of  these 
again  divides  into  two,  and  this  continues  for  generation 
after  generation.  Nowhere  does  a  corpse  appear;  nothing 


80  Life  and  Death,  Heredity  and  Evolution 

dead  is  found  in  the  entire  history  of  a  race,  save  by  accident. 
The  present  living  generation  transforms  directly  into  the 
next  one.  Figure  5  illustrates,  the  course  of  life  and  the 
passage  of  generations  in  these  creatures  as  compared  with 
that  of  higher  organisms. 

All  this  is  so  different  from  what  we  ourselves  experience 


Figure  4.    Division  of  an  infusorian,  Paramecium.    Successive  stages. 

and  from  what  we  see  around  us  in  the  animals  that  we  know, 
that  it  has  always  aroused  the  greatest  interest  and  led  to 
many  questions.  What !  are  these  creatures  really  im- 
mortal? Is  there  no  decline,  no  old  age,  no  natural  death 
in  their  history?  Does  their  life  realize  the  dream  of 
perpetual  youth?  Such  questions,  as  you  know,  were  put 
long  ago  by  the  early  students  of  these  creatures,  and  for 


General  Survey  £1 

almost  a  century  the  answers  have  been  sought,  through 
speculation  and  reasoning,  through  observation  and  experi- 
ment. After  much  conflict  of  opinion  and  many  changes 


Figure  5.  Diagram  of  the  descent  of  generations  in  Protozoa  or 
other  organisms  reproducing  from  a  single  parent;  as  compared  with 
that  in  organisms  reproducing  from  two  parents.  The  lines  represent 
the  lives  of  individuals  or  germ  cells,  beginning  at  the  left  and  passing 
to  the  right.  A,  uniparental  reproduction  by  fission.  The  line  of 
ancestry  traced  back  from  any  individual  at  the  right  is  always  single; 
and  there  is  no  corpse  found  at  any  point,  the  present  body  transform- 
ing directly  into  the  bodies  of  the  next  generation.  B,  biparental  repro- 
duction by  union  of  germ  cells,  as  in  the  higher  organisms.  The  tri- 
angular structures  are  the  bodies,  the  lines  the  germ  cells.  The  line  of 
ancestry  from  any  individual  traced  back  from  the  right  forks  at  each 
generation,  becoming  in  a  few  generations  multiplex.  The  bodies  of 
any  generation  are  not  continuous  with  the  bodies  of  the  previous  gen- 
eration— the  latter  dying,  while  the  new  bodies  are  produced  by  germ 
cells  from  two  diverse  lines. 


22  Life  and  Death,  Heredity  and  Evolution 

in  the  prevalent  views,  I  believe  that  the  last  few  years  have 
brought  us  the  facts  which  answer  these  questions.  It  is 
of  these  that  I  wish  first  to  speak;  they  form  a  foundation 
for  all  our  knowledge  of  these  matters  in  Protozoa. 

Although  it  appears  that  these  creatures  do  not  naturally 
die,  it  is  equally  apparent  that  they  do  reproduce.  Every 
day  or  two,  or  even  more  frequently,  each  individual  divides, 
and  we  have  two  where  there  was  but  one;  in  a  few  days 
their  number  has  multiplied  enormously.  And  if  we  watch 
them  long  and  closely  enough,  we  make  another  discovery 
of  great  interest:  after  the  passage  of  many  generations 
these  creatures  mate,  as  do  higher  animals  and  plants.  Two 
individuals  unite  and  exchange  parts  of  their  bodies  (figure 
6),  then  separate  and  both  continue  to  live  and  to  reproduce 
as  before. 


Figure  6.     Conjugation  of  Paramecium. 

Now  if  these  creatures  do  not  die,  why  should  they  mate 
and  why  should  they  reproduce?  In  ourselves  and  in  the 
animals  we  know  intimately,  mating  and  reproduction  are 
the  prelude  to  death;  reproduction  seems  a  method  of  re- 
placing the  individuals  that  are  to  die.  What  is  the  use 
of  these  processes  if  those  now  alive  are  to  continue  to 


Theory  of  Rejuvenescence  28 

exist?  Suppose  that  there  were  no  death  in  man;  there 
would  be  no  need  for  reproduction;  and  if  reproduction 
occurred  it  must  soon  result  in  overcrowding  and  in  violent 
killing  off  on  even  a  greater  scale  than  occurred  in  the 
great  war.  And  in  these  simple  creatures  the  same  over- 
crowding must  result,  for  reproduction  every  twenty-four 
hours,  without  death,  would  in  a  few  months  pack  all  the 
waters  of  the  earth  with  a  solid  mass  of  these  creatures. 

Reflecting  thus  on  these  things,  many  biologists  came  to 
the  conclusion  that  these  creatures  must  get  old  and  die 
after  all ;  that  otherwise  they  would  not  mate  and  reproduce. 
We  see  indeed  that  not  every  individual  gets  old,  for  they 
continue  to  live  and  reproduce  for  generation  after  genera- 
tion. So  it  was  thought  that  it  must  be  the  passage  of 
many  generations  that  brings  on  age.  The  creatures,  it 
was  held,  begin  young  and  strong;  they  divide  again  and 
again,  gradually  getting  old  and  weak;  it  is  only  the  indi- 
viduals of  these  later  generations  that  show  the  weakness  of 
age. 

Now,  since  in  higher  animals  mating  and  reproduction 
bring  it  about  that  an  old  and  worn  individual  is  replaced  by 
one  young  and  unworn,  so,  it  was  reasoned,  must  mating  in 
these  lower  creatures  cause  an  old  individual  to  be  replaced 
by  a  new  one.  But  .when  we  observe  the  process,  we  do  not 
see  a  new  individual  replace  an  old  one  after  mating;  ap- 
parently the  old  one  continues  to  live  and  multiply  as  before. 
To  avoid  this  difficulty  it  was  concluded  that  in  these  crea- 
tures mating  must  rejuvenate  the  old  individual;  must  in 
some  way  get  rid  of  the  age  and  wear,  leaving  the  same 
individual,  but  physically  young. 

This  was  the  famous  theory  of  rejuvenescence  through 
conjugation.  It  held  that  the  young  animal  may  live  and 
reproduce  for  many  generations  without  mating,  till  thou- 


24>  Life  and  Death,  Heredity  and  Evolution 

sands  of  specimens  have  been  produced  from  a  single  one. 
But  as  this  goes  on,  they  gradually  lose  their  vitality,  get 
old  and  decrepit,  and  must  all  in  the  course  of  time  die, 
unless  mating  intervenes  to  save  them.  On  the  question  of 
just  what  form  this  aging  and  decrepitude  takes  and  just 
what  are  its  symptoms,  there  was  much  difference  of  opinion. 
Biitschli  and  Balbiani  believed  that  the  power  of  reproduc- 
tion gradually  became  less  and  less,  so  that  division  into 
two  became  less  frequent;  this  idea  was  accepted  by  many, 
and  has  been  maintained  in  recent  years  by  Calkins;  it  is 
perhaps  the  prevalent  form  of  the  theory.  On  this  view  the 
occurrence  of  conjugation  restored  the  reproductive  power, 
so  that  fission  now  continued  as  rapidly  as  it  did  before  the 
animals  grew  old.  On  the  other  hand,  though  the  fact  is 
not  generally  realized,  some  of  the  chief  upholders  of  the 
theory  of  rejuvenescence  emphatically  rejected  this  idea 
that  aging  showed  itself  in  a  decline  of  the  rate  of  repro- 
duction ;  this  is  notably  the  case  with  Maupas,  who  asserted 
positively  that  no  such  decline  occurred  before  conjugation, 
and  that  after  conjugation  reproduction  was  no  more  ener- 
getic than  before;  and  Richard  Hertwig  discovered  experi- 
mentally that  Maupas'  statement  is  correct.  They  believed 
however  that  in  other  ways  the  animals  become  decrepit  and 
that  they  must  die  if  conjugation  did  not  occur. 

Now,  just  what  is  it  that  happens  in  mating  which  might 
be  conceived  to  bring  about  rejuvenescence?  There  are  two 
main  things  that  occur  in  conjugation;  to  get  these  clearly 
in  mind  we  must  look  for  a  moment  at  the  structure  of  an 
infusorian,  and  at  the  process  of  conjugation. 

An  infusorian  resembles  in  the  plan  of  its  structure  a 
single  cell  of  a  higher  organism,  but  has  its  nucleus  in  two 
parts  (Figure  7,  a).  One  of  these  parts  is  large  and  seems 
to  be  the  active  portion ;  the  other  is  very  minute  and  appears 


Conjugation  25 

to  be  a  sort  of  reserve  of  nuclear  material.  The  large  part, 
or  macronucleus,  seems  to  take  a  part  in  the  physiological 
processes  of  the  daily  life  of  the  cell.  The  reserve  nucleus, 
or  micronucleus,  seems  to  lie  inactive,  save  at  the  time  when 
the  creature  divides ;  then  both  the  large,  active  nucleus  and 
the  small  reserve  nucleus  divide,  so  that  both  the  new  indi- 


— c  v 


ma 


c  v 


Figure  7.  Diagram  of  the  process  of  division  in  Paramecium;  suc- 
cessive stages,  c.  v.,  Contractile  vacuoles;  m.,  Mouth;  ma.,  Macro- 
nucleus;  mi.,  Micronucleus. 


viduals  produced  contain  half  of  each   (Figure  7,  c,  d). 
Sometimes  there  are  several  of  the  small  reserve  nuclei. 

Now  at  the  time  of  conjugation  (Figure  8),  in  each  of  the 
mated  individuals  the  old  active  nucleus  breaks  up  and 
gradually  disappears,  being  apparently  absorbed  like  so 
much  food,  by  the  rest  of  the  body.  The  reserve  nucleus 
(mi.,  Figure  8),  on  the  other  hand,  divides  several  times; 
in  Paramecium  caudatum,  for  example,  each  divides  twice 


• 

26  Life  and  Death,  Heredity  and  Evolution 

(A  to  C),  so  that  four  are  produced.  Three  of  these  dis- 
appear, as  did  the  old  active  nucleus,  and  the  fourth  divides 
into  two  halves  (D).  One  of  these  halves  passes  over  into 


Figure  8.  Diagram  of  the  internal  processes  in  the  conjugation  of 
Paramecium  caudatum;  successive  stages,  ma.,  Macronucleus ;  mi., 
Micronucleus.  The  striated  and  spindle-shaped  bodies  are  the  micro- 
nucleus  in  the  process  of  division.  In  D  the  three  smaller  dotted  clouds 
are  three  degenerating  micronuclei. 

the  other  individual,  with  which  mating  is  occurring  (Figure 
8,  E),  where  it  unites  with  the  corresponding  remaining  half 
of  the  reserve  nucleus  of  that  individual  (F,  G).  That  is, 
the  two  conjugating  animals  exchange  halves  of  their  reserve 


Conjugation  27 

nuclei,  so  that  each  has  after  mating  a  new  reserve  nucleus, 
composed  half  of  reserve  nuclear  material  from  its  own 
body,  half  of  reserve  nuclear  material  from  the  body  of  its 
mate  (Figure  8,  G.).  Later  this  new  reserve  nucleus  divides 
into  parts,  some  of  which  become  new,  large,  active  macro- 
nuclei,  while  the  others  remain  as  minute  micronuclei  (see 
Figure  30,  Lecture  5). 

So  there  are  two  main  things  in  conjugation:  (1)  The 
old  active  nucleus  is  replaced  by  parts  of  the  reserve  nuclei ; 
(2)  the  two  mating  individuals  exchange  parts  of  their 
nuclear  material. 

Now,  evidently  the  replacement  of  the  old  active  nucleus 
by  part  of  the  reserve  nucleus  is  just  the  sort  of  thing  that 
one  would  expect  if  there  is  to  be  rejuvenescence;  indeed,  it 
is  rejuvenescence  of  the  macronucleus.  It  Jooks  very  much 
as  if  the  old  active  nucleus  might  have  gotten  worn  out  or 
used  up  in  its  activity,  so  that  it  has  to  be  replaced  by 
reserve  material  which  has  not  been  used. 

But  what  has  all  this  to  do  with  mating?  If  the  point 
is  merely  the  replacement  of  worn  nuclear  material  by  fresh 
material  from  the  reserve  store  that  each  animal  carries, 
why  need  there  be  this  complicated  process  of  exchange  of 
nuclei?  Rejuvenescence  should  occur  just  as  well  without 
this  exchange,  without  conjugation,  as  with  it. 

The  occurrence  of  these  two  distinct  things  at  conjuga- 
tion,— replacement  from  a  reserve,  and  exchange, — has  al- 
ways kept  the  theory  of  rejuvenescence  ambiguous.  Is  it 
the  replacement  from  reserve  material,  or  the  exchange,  that 
makes  the  organism  young  again?  Authors  have  as  a  rule 
either  not  ventured  to  answer  this  question,  or  have  not 
clearly  analyzed  the  process  into  its  two  elements, — speaking 
merely  of  conjugation  as  a  whole.  Arguments  based  on  the 
replacement  process  have  been  used  or  accepted  as  argu- 


28  Life  and  Death,  Heredity  and  Evolution 

ments  referring  to  the  process  of  exchange,  and  vice  versa. 
What  was  needed  was  some  method  of  separating  the  two 
processes,  to  see  what  effect  each  has  by  itself. 

In  the  meantime,  taking  conjugation  as  it  occurs,  without 
this  analysis,  men  attempted  to  find  out:  first,  whether  the 
organisms  do  grow  old  and  die  if  conjugation  does  not 
occur;  second,  whether  conjugation  does  save  them,  does 
make  them  young,  does  cause  them  to  reproduce  more  ener- 
getically. 

The  first  question,  as  to  whether  without  conjugation  the 
creatures  degenerate  and  die,  has  cost  an  infinite  amount 
of  labor  to  generations  of  investigators.  It  appeared  that 
as  a  matter  of  fact,  if  the  animals  were  kept  without  con- 
jugation they  do  die  out  in  the  course  of  time;  such  was 
the  result  of  the  long  continued  labors  of  Maupas  (1888, 
1889).  After  some  hundreds  of  generations  without  con- 
jugation the  animals  weakened,  became  abnormal,  sickened 
and  died.  It  is  worth  while  to  note  Maupas'  exact  results 
on  this  point.  We  may  give  them  in  the  words  of  his  own 
summary : — 

"I  have  kept  six  cultures  until  their  final  extinction  by 
senile  exhaustion.  The  first  (Stylonychia  pustulata)  be- 
came extinct  after  215  fissions;  the  second  (Stylonychia 
pustulata)  after  316;  the  third  (Stylonychia  mytilus)  after 
319;  the  fourth  ( Onychodromus  grandis)  after  320  to  330; 
the  fifth  (Oxytricha)  after  320  to  330;  the  sixth  finally 
(Leucophrys  patula)  after  660."  (Maupas  1888,  p.  260). 1 

But  as  other  investjgators  took  up  the  same  sort  of  work, 
a  curious  fact  was  found.  As  men  began  to  be  able  to 

*It  is  worth  noting  that  different  infusoria  showed  great  diversities 
as  to  length  of  life;  Leucophrys  patula  lived  in  Maupas'  experiments 
for  twice  as  many  generations  as  any  of  the  other  species.  It  should 
also  be  observed  that  Maupas  did  not  carry  out  such  experiments  on 
Paramecium,.  the  organism  that  has  been  most  used  for  such  work  since 
his  time. 


Conjugation  89 

take  better  care  of  the  animals;  to  give  them  proper  food, 
and  to  vary  their  food,  the  organisms  were  found  to  live 
longer  and  longer  without  conjugation,  and  to  give  less 
indication  of  old  age.  Calkins  (1904)  kept  the  infusorian 
Paramecium  caudatum  for  742  generations  without  conjuga- 
tion, but  they  finally  weakened  and  died.  Enriques  (1903) 
kept  Glaucoma  scintillans  for  683  generations  without  con- 
jugation and  with  no  sign  of  harmful  effects;  and  he  has 
recently  (1916)  kept  Glaucoma  pyriformis  for  2701  genera- 
tions with  no  sign  of  degeneration.  And  finally  Woodruff 
(1917)  has  kept  Paramecium  aurelia  for  more  than  6000 
generations  without  conjugation;  the  stock  at  last  accounts 
was  still  flourishing,  with  no  sign  of  aging,  of  degeneration. 

Now  the  fact  that  one  can  by  proper  methods  of  culture 
keep  these  creatures  healthy  for  more  than  6000  generations 
without  conjugation,  shows  that  the  degeneration  which 
came,  under  other  methods  of  culture,  in  a  few  hundred 
generations,  was  not  evidence  that  conjugation  is  required, 
but  only  that  the  culture  methods  were  not  good.  It  has 
been  found  that  some  species  of  infusoria  cannot  stand,  save 
for  a  short  time,  the  method  of  culture  necessary  if  the 
separate  generations  are  to  be  accurately  counted;  others 
can  exist  under  these  conditions  for  a  greater  number  of 
generations;  other  indefinitely;  and  that  without  conjuga- 
tion. 

Thus  the  result  of  the  work  so  far  done  has  been  to  con- 
firm the  view  that  the  infusoria  may  live  indefinitely  without 
mating.  I  believe  that  we  may  look  upon  this  as  one  of 
the  secure  results  of  science.  There  are  many  of  the  uni- 
cellular creatures,  particularly  the  bacteria,  in  which 
nothing  like  mating  is  known.  It  is  sometimes  held  that 
such  processes  must  yet  be  found  in  these  creatures.  But 
the  fact  that  infusoria,  which  do  mate,  may  nevertheless  live 


30  Life  and  Death,  Heredity  and  Evolution 

indefinitely  without  mating,  makes  it  probable  that  the  ap- 
pearances are  correct,  and  that  such  organisms  as  bacteria 
actually  never  mate. 

Does  this  confirm  the  theory  of  the  "potential  immortal- 
ity" of  these  creatures?  It  apparently  does,  if  we  are  to 
take  that  expression  in  its  broadest  meaning.  But  the  real 
question  underlying  that  phrase  is  this:  Does  the  exercise 
of  the  functions  of  life  itself  necessarily  result  in  deteriora- 
tion, in  senility,  in  final  death?  This  was  the  question  that 
Maupas  believed  his  experiments  to  answer  affirmatively. 
He  says  of  his  experiments,  "They  demonstrate  that  indeed 
in  the  ciliate  infusoria,  as  in  so  many,  if  not  all,  other  living 
things,  the  organism  deteriorates,  uses  itself  up,  simply  by 
the  prolonged  exercise  of  its  functions"  (1888,  p.  261). 

The  experiments  we  have  described  above  show  that  con- 
jugation is  not  required  to  remedy  the  wearing  away  of  the 
organism  through  the  exercise  of  its  vital  functions.  But 
is  this  the  complete  story?  Is  there  after  all  no  significance 
in  the  fact  that  these  creatures  keep  a  reserve  nucleus,  along 
with  the  active  one?  Does  the  active  nucleus  never  require 
replacement  from  the  reserve? 

This  question  too  has  recently  been  answered,  mainly  by 
the  work  of  Woodruff  and  Erdmann  (1914).  And  in  an- 
swering the  question  they  have  succeeded  in  observing  that 
separation  of  the  two  processes  in  conjugation,  of  which 
we  said  above  there  was  so  much  need.  These  two  processes 
are:  first,  the  replacement  of  the  active  nucleus  from  the 
reserve;  second,  the  exchange  of  parts  of  the  nuclei  in 
mating.  Woodruff  and  Erdmann  observed  the  replacement 
process  occurring  without  mating  and  exchange.  And  in 
doing  this  they  have  uncovered  what  is  evidently  one  of  the 
most  important  phenomena  that  occur  in  the  life  of  these 
Protozoa;  something  that  must  form  a  background  for  all 


Endomixis 


31 


study  of  life  and  reproduction  in  these  creatures.     We  must 
therefore  examine  it  with  care. 

In  their  race  of  Paramecium  that  lived  indefinitely  with- 
out conjugation,  Woodruff  and  Erdmann  found  that  the 
active  macronucleus  is  replaced  at  rather  short  intervals 


Figure  9.  Diagram  showing  what  occurs  in  endomixis,  or  the  replace- 
ment of  the  old  active  macronucleus  by  a  part  of  the  reserve  nucleus 
(micronucleus)'  in  Paramecium  aurelia.  Constructed  from  the  figures 
and  description  of  Woodruff  and  Erdmann. 

The  earliest  stage  (condition  before  the  process  begins)  is  shown  at 
the  top ;  successive  later  stages  from  above  downward.  The  larger  black 
bodies  represent  the  macronuclei;  the  smaller  ones  the  micronuclei.  The 
clear  circles  are  the  micronuclei  that  degenerate. 


32  Life  and  Death,  Heredity  and  Evolution 

by  parts  of  the  reserve  nucleus.  Every  forty  to  fifty  gen- 
erations the  macronucleus  breaks  up  and  is  absorbed  and 
disappears,  just  as  happens  when  conjugation  is  to  occur. 
Then  each  reserve  nucleus  divides  (in  Paramecium  aurelia) 
so  as  to  produce  eight  (see  Figure  9).  Six  (or  seven) 
of  these  disappear  by  absorption,  like  the  active  nucleus. 
The  remaining  one  later  produces  by  division  the  new  active 
nucleus  (macronucleus),  and  the  reserve  nucleus  or  nuclei. 
That  is,  the  active  nucleus  is  replaced  every  forty  or  fifty 
generations  by  material  from  the  reserve  store.  This  whole 
process  Woodruff  and  Erdmann  call  endomixis. 

This  process  has  been  found  to  occur,  not  only  in  the 
single  race  in  which  Woodruff  and  Erdmann  first  found  it, 
but  in  many  other  races,  and  in  another  species  of  Para- 
mecium. And  in  another  infusorian,  Stylonychia,  the  Rus- 
sian investigator,  Fermor  (1913),  states  that  a  similar  proc- 
ess occurs  at  the  time  of  encystment.  After  multiplying 
for  five  or  six  weeks  the  animals  lose  all  their  appendages 
and  other  organs,  gather  into  a  sphere  and  form  a  sort 
of  thin  shell  about  them.  Then  the  two  active  nuclei  disap- 
pear and  are  replaced  from  the  reserve  nuclei,  which  have 
united  to  form  one.  The  very  brief  account  of  this  process 
by  Fermor  was  published  earlier  than  the  work  of  Woodruff 
and  Erdmann.  It  has  been  stated  by  Calkins  that  a  similar 
process  occurs  in  the  encystment  of  Didinium  (1915  a),  and 
in  Uroleptus  (1919).  It  seems  probable  that  it  will  be  found 
generally  in  the  infusoria. 

The  discovery  of  this  process  of  replacement  of  the  active 
nucleus  by  the  reserve  nucleus  evidently  puts  a  new  face 
on  the  matter.  To  the  question  whether  the  living  sub- 
stance uses  itself  up  in  functioning,  so  as  to  require  replace- 
ment, it  seems  to  answer  "yes !"  even  more  definitely  than  any 
discovery  that  conjugation  was  necessary  would  have  done. 


Endomixis  33 

The  question  becomes  of  extreme  interest  whether  such  a 
process  as  this  is  of  general  occurrence ;  whether  it  is  neces- 
sary in  order  that  aging  and  death  shall  not  occur.  Many 
investigations  are  therefore  at  this  time  directed  upon 
endomixis.  Certain  questions  must  occur  to  everyone ;  some 
of  these  have  already  been  answered. 

First,  is  this  replacement  perhaps  merely  something 
brought  about  by  unfavorable  conditions  of  the  environment, 
and  not  necessary  if  all  conditions  continue  good?  Jollos 
(1916),  Young  (1917),  and  Woodruff  (1917)  find  that 
under  unfavorable  conditions  the  replacement  of  the  active 
nucleus  from  the  reserve  is  brought  about  more  quickly  than 
would  otherwise  occur.  But  under  the  most  favorable  con- 
ditions the  process  does  not  cease,  and  under  uniform  con- 
ditions it  takes  place  at  uniform  intervals.  Woodruff  found 
that  whenever  the  process  of  replacement  ceases  entirely,  the 
race  dies  out.  This  evidence  therefore  indicates  that  the 
process  is  a  necessity  for  continued  life. 

Enriques  (1916)  attempted  to  test  the  matter  by  studying 
an  organism  in  which  there  were  no  periods  of  slow  fission, 
such  as  are  found  while  the  replacement  is  occurring.  He 
found  such  an  infusorian  in  Glaucoma  pyriformis,  which 
under  the  conditions  he  used  may  produce  10  to  13  genera- 
tions a  day,  and  continue  this  without  interruption.  He 
kept  this  organism  for  2701  generations,  or  for  more  than 
8  months,  during  which  there  were  not  less  than  9  genera- 
tions every  single  day.  The  culture  was  still  in  progress 
at  last  accounts ;  there  was  no  sign  of  degeneration,  and 
no  periods  of  slow  fission.  Enriques  therefore  holds  that 
endomixis  has  not  occurred.  He  concludes  that  endomixis 
is  not  necessary  to  continued  life,  just  as  conjugation  is  not. 

It  is  evident  however  that  this  conclusion  is  insecure;  it 
is  not  impossible  that  endomixis  occurred  without  interrupt- 


34  Life  and  Death,  Heredity  and  Evolution 

ing  the  regular  series  of  fissions.  The  matter  requires  actual 
cytological  study  before  we  can  know  whether  or  not  any- 
thing of  the  sort  occurs  in  Glaucoma. 

In  Paramecium,  Woodruff  and  Erdmann  (1914)  found 
that  while  the  replacement  is  going  on,  and  for  a  little  before 
and  after,  the  rate  of  reproduction  is  slower  than  at  other 
times.  Does  this  mean  a  waning  of  vitality,  which  is  cor- 
rected by  the  replacement?  Jollos  (1916)  found  that  under 
favorable  and  uniform  conditions,  the  slowing  of  the  fission 
rate  is  limited  sharply  to  the  period  in  which  the  processes 
of  replacement  are  occurring.  This  slowing  of  cell  division 
would  therefore  appear  to  be  merely  a  natural  result  of  the 
fact  that  while  the  complicated  changes  of  endomixis  are 
occurring,  the  organism  does  not  so  readily  undergo  at 
the  same  time  the  involved  processes  of  fission.  It  is  there- 
fore not  clear  that  there  is  any  indication  of  loss  of  vigor, 
of  senile  changes,  setting  in  before  each  replacement  occurs. 
The  process  seems  to  be  carried  on,  like  many  others  in 
nature,  with  such  a  "margin  of  safety"  that  there  is  no 
indication  of  exhaustion  before  it  occurs;  it  takes  place  be- 
fore there  is  a  pressing  need  for  it. 

Thus  it  appears  that  in  these  organisms  nature  has  em- 
ployed the  method  of  keeping  on  hand  a  reserve  stock  of  a 
material  essential  to  life ;  by  replacing  at  intervals  the  worn 
out  material  with  this  reserve,  the  animals  are  kept  in  a  state 
of  perpetual  vigor ;  not,  as  individuals,  growing  old  or  dying 
a  natural  death.  Nevertheless,  a  wearing  out  process,  such 
as  might  be  called  getting  old,  does  occur  in  the  structures 
employed  in  the  active  functions  of  life,  and  these  must  be 
replaced  after  a  time  of  service.  So  far  as  the  conditions 
in  these  organisms  are  typical,  deterioration  and  death  do 
appear  to  be  a  consequence  of  full  and  active  life ;  life  carries 
within  itself  the  seeds  of  death.  It  is  not  mating  with  an- 


Continuity  of  Life  35 

other  individual  that  avoids  this  end ;  but  replacement  of  the 
worn  material  by  a  reserve. 

It  results  that  the  continuity  of  life  in  the  infusoria  is  in 
principle  much  like  that  in  ourselves,  though  with  differences 
in  details.  As  individuals,  the  infusoria  do  not  die,  save  by 
accident.  Those  that  we  now  see  under  our  microscopes 
have  been  living  ever  since  the  beginnings  of  b'fe ;  they  come 
from  division  of  previously  existing  individuals.  But  in  just 
the  same  sense,  it  is  true  for  ourselves  that  everyone  that 
is  alive  now  has  been  alive  since  the  beginning  of  b'fe.  This 
truth  applies  at  least  to  our  bodies  that  are  alive  now ;  every 
cell  of  all  our  bodies  is  a  piece  of  one  or  more  cells  that 
existed  earlier,  and  thus  our  entire  body  can  be  traced  in 
an  unbroken  chain  as  far  back  into  time  as  life  goes  (see 
the  diagram,  Figure  5).  The  difference  is  that  in  man 
and  other  higher  organisms  there  have  been  left  all  along 
the  way  great  masses  of  cells  that  did  not  continue  to  live. 
These  masses  that  wore  out  and  died  are  what  we  call  the 
bodies  of  the  persons  of  earlier  generations;  but  our  own 
bodies  are  not  descended  by  cell  division  from  these;  they 
are  the  continuation  of  cells  that  have  kept  on  living  and 
multiplying  from  the  earliest  times,  just  as  have  the  existing 
infusoria.  From  our  own  personal  point  of  view  it  seems 
unfortunate  that  the  mass  of  cells  which  is  next  to  wear  out 
and  be  left  behind  in  the  chain  of  life  is  that  with  which  our 
own  selves  seem  to  be  bound  up ;  but  certain  samples  of  our- 
selves may  continue  to  live  indefinitely,  like  the  infusorian. 
The  great  mass  of  cells  subject  to  death  in  the  higher 
animals  dwindles  in  the  infusorian  to  the  macronucleus ;  this 
alone  represents  a  corpse.  But  the  dissolution  of  this  corpse 
occurs  within  the  living  body.  It  resembles  much  such  a 
process  as  the  wasting  away  and  destruction  of  minute  parts 
of  our  own  bodies,  which  we  know  is  taking  place  at  all 


36  Life  and  Death,  Heredity  and  Evolution 

times  and  which  does  not  interrupt  the  life  of  the  individual. 

What  now  are  we  to  think  of  conjugation  in  the  light  of 
these  new  facts?  In  conjugation  there  occurs  the  same  re- 
placement of  the  old  active  nucleus  by  a  part  of  the  reserve, 
which  we  have  seen  to  take  place  also  without  conjugation ; 
but  with  the  additional  fact  of  an  exchange  of  pieces  of  the 
new  nucleus  between  the  two  conjugating  individuals.  If 
this  replacement  means  rejuvenescence  when  it  occurs  with- 
out conjugation,  there  is  no  reason  to  deny  it  that  meaning 
in  conjugation;  conjugation  too  should  result  in  rejuvenes- 
cence. But  what  is  the  significance  of  the  additional  feature 
— the  exchange  of  nuclear  parts  between  the  two  conjugat- 
ing animals?  This  seems  not  necessary  to  rejuvenescence, 
since  the  replacement  of  the  old  by  the  reserve  may  occur 
without  it.  Why  then  does  the  exchange  take  place? 

In  many  higher  organisms  we  observe  that  the  mating 
and  the  union  of  parts  of  two  different  cells  which  we  call 
fertilization  are  necessary  in  order  that  reproduction  shall 
occur;  the  egg  does  not  develop  unless  fertilized.  But  in 
these  Protozoa  we  observe  that  reproduction  occurs  for  gen- 
eration after  generation  without  this  mixture  of  two  diverse 
cells;  and  we  have  just  seen  that  rejuvenescence  likewise 
occurs  without  such  a  mixture.  In  these  lower  creatures  we 
find  separated  therefore  two  processes  which  in  many  higher 
animals  are  so  closely  bound  together  that  we  get  the  im- 
pression that  they  are  inseparable;  that  development  neces- 
sarily depends  on  fertilization.  But  even  in  many  higher 
animals  development  may  take  place  without  fertilization; 
these,  taken  with  the  facts  in  the  Protozoa,  show  that  there 
is  no  generally  necessary  relation  between  the  two  things. 
It  appears,  as  we  shall  see  later,  that  the  close  association 
of  reproduction  with  a  union  of  two  cells  is  only  a  special 
peculiarity  of  certain  organisms:  something  that  might  be 
called  a  special  adaptation. 


What  is  the  Result  of  Mating  37 

What  ground  then  can  we  possibly  give  for  the  inter- 
change of  parts  of  the  two  individuals  that  conjugate?  We 
shall  take  up  this  question  in  all  its  aspects  later;  here  I 
wish  to  bring  up  but  one  of  the  possibilities.  We  observe 
that  after  the  two  individuals  have  conjugated  and  sep- 
arated, they  are  no  longer  just  what  they  were  before.  Each 
is  now  formed  of  parts  of  two  individuals, — a  body  and  half 
the  nucleus  from  one;  half  the  nucleus  from  the  other. 
Will  the  two  individuals  therefore  now  be  diverse  in  other 
respects  from  what  they  were  before?  Will  their  general 
characteristics  be  changed?  Will  they  behave  differently; 
will  they  develop  differently;  will  they  produce  young  of  a 
different  sort?  In  other  words,  are  the  other  characteristics 
of  the  two  individuals  mixed  as  well  as  their  nuclei? 

This  we  know  is  what  happens  as  a  result  of  fertilization 
in  higher  organisms;  the  young  produced  inherit  from  both 
parents.  Does  it  happen  also  in  the  Protozoa?  If  so,  it 
will  give  us  an  understanding  of  the  exchange  of  parts  in 
mating. 

This  raises  for  us  the  problem  of  heredity  in  the  Protozoa. 
Do  the  young  produced  by  any  given  parent  inherit  also 
from  the  individual  with  which  that  parent  has  mated? 

But  this  is  heredity  in  its  most  complex  form.  In  the 
Protozoa,  as  we  have  seen,  we  have,  for  generation  after 
generation,  reproduction  from  a  single  parent  by  simple 
division.  This  presents  the  problem  of  heredity,  and  also 
those  of  variation  and  evolution,  in  the  simplest  possible 
form,  and  we  shall  do  well  to  study  the  problems  here 
before  we  take  them  up  in  cases  where  two  parents  are 
involved.  We  shall  therefore  examine  this  matter  in  our 
next  lecture,  and  later  take  up  the  entire  natural  history  of 
mating. 


II 


Heredity  and  Variation  in  Protozoa,  in  Reproduction  from 
a  Single  Parent.  "The  Inheritance  of  Acquired  Characters." 
The  Existence  of  Many  Diverse  Stocks  in  a  Single  Species. 
Constancy  of  the  Stocks.  "Pure  Line  Inheritance"  and  the 
Results  of  Selection.  Difficulties  for  the  Theory  of  Evolu" 
tion. 

R  first  chapter  led  us  to  the  question  of  heredity  in 
the  Protozoa.  In  the  present  chapter  we  take  up 
the  study  of  heredity  and  variation  in  the  simplest  kind  of 
reproduction,  where  the  offspring  are  produced  by  the  divi- 
sion of  a  single  individual  into  two.  By  way  of  introduction, 
let  us  bring  this  into  relation  with  the  problem  of  life  and 
death,  which  we  have  already  considered.  We  have  seen 
that  these  organisms  are  so  constituted  that  they  live  in- 
definitely, with  no  intervention  of  natural  death  of  the  in- 
dividuals ;  yet  we  have  seen  also  that  they  continue  to 
reproduce.  The  inevitable  result  is  that  more  individuals 
are  produced  in  each  species  than  nature  can  provide  space 
and  opportunity  for,  so  that  most  of  them  are  condemned 
to  violent  and  unnatural  death.  Is  there  anything  changed 
by  this  continual  over-production,  with  destruction  of  the 
majority?  Are  those  produced  exactly  like  those  that 
existed  before?  Or  do  the  animals  change  as  generations 
pass,  so  that  some  are  better  fitted  for  the  conditions  that 
they  meet,  and  therefore  continue  to  exist,  while  others  are 
killed  off?  That  is,  can  we  see  evolution  occur  as  we  watch 
these  creatures  through  generation  after  generation? 
38 


Heredity  in  Protozoa  39 

Turning  now  to  the  facts,  we  know  that  when  one  of 
these  animals  reproduces,  it  merely  divides  into  two;  the 
parent  simply  transforms  into  two  offspring.  Are  the  two 
just  like  the  parent  from  which  they  came?  When  we 


Figure  10.    Reproduction  in  Amoeba;  successive  stages.    After  F.  E. 
Schulze. 


examine  the  facts  in  amaba  (Figure  10),  we  can  see  no 
reason  why  they  should  not  be ;  they  merely  are  the  parent, 
but  now  in  two  parts;  the  difference  in  size  between  the 
formerly  existing  parent  and  its  two  progeny  is  quickly 
remedied  by  a  little  growth.  So  at  first  view  the  problem 
of  heredity  seems  in  these  creatures  absolutely  simple;  the 
progeny  are  the  parents,  merely  divided;  so  they  must  be 
like  the  parents. 

But  if  we  don't  stop  at  amoeba,  but  examine  other  Proto- 
zoa also,  we  find  that  the  matter  is  not  so  entirely  simple 
after  all;  indeed,  we  find  that  heredity  presents  the  same 
problems  that  it  does  in  higher  animals.  Take  for  example 


40  Life  and  Death,  Heredity  and  Evolution 

a  close  relative  of  amoeba, — Difflugia  (Figure  11),  which 
is  merely  an  amoeba  with  a  shell.  In  reproduction  the  two 
products  do  not  receive  half  the  parental  shell;  if  they  did, 
as  you  can  readily  see,  they  would  indeed  not  be  like  the 


Figure  11.     Reproduction  in  Difflugia.     After  Verworn    (1888). 

parent.  What  happens  is  this:  The  shell  consists  of  sand 
grains,  embedded  in  a  hard  chitinous  substance.  As  the 
parent  creeps  about  in  its  daily  life,  it  takes  up  sand  grains 
and  stores  them  in  the  interior  of  its  body.  At  reproduction 
the  protoplasm  of  the  parent  swells  and  projects  from  the 
mouth  of  its  shell  (Figure  11,  A).  This  projecting  mass 
takes  a  form  similar  to  that  of  the  parent  (B).  The  sand 
grains  within  the  parent  body  pass  out  into  the  projecting 
mass,  come  to  its  surface,  and  spread  over  it  (C,  D).  They 
are  embedded  in  a  fluid  secretion  which  now  turns  hard,  so 
that  they  form  a  shell  like  that  of  the  parent.  The  two. 
shells  are  in  contact  at  their  mouths  (D).  Now  the  common 
mass  of  protoplasms  divides  into  two,  and  the  two  individuals 
separate — one  retaining  the  old  shell;  the  other  with  the 
new  one. 

Now  you  see  that  it  is  by  no  means  a  simple  matter  of 


Heredity  m  Protozoa 


41 


course  that  the  new  individual  should  have  a  shell  just  like 
that  of  the  parent.  Its  shell  is  produced  anew.  The  parent 
shell  may  have  a  peculiar  form  or  structure;  in  some  cases 


Figure  12.  Two  parents,  with  their  offspring,  just  before  separation, 
in  Difflugia  corona.  The  parents  are  above,  the  offspring  (slightly 
lighter  in  shade)  below.  After  Jennings,  1916. 

for  example,  as  you  see  in  Figure  1£,  it  bears  spines  of  a 
certain  length  and  form  and  in  a  certain  number.  There  is 
no  simple  evident  reason  why  the  progeny  should  produce 
a  shell  of  the  same  form,  with  spines  of  the  same  length  and 
number.  These  things  are  not  by  any  means  merely  handed 
on  bodily  from  parent  to  offspring.  On  the  contrary,  just 


42  Life  and  Death,  Heredity  and  Evolution 

as  in  higher  animals  and  ourselves,  the  progeny  produce 
anew  their  peculiar  characteristics.  The  problem  of  heredity 
is : — Why  should  they  produce  the  same  sort  of  characteris- 
tics that  their  parents  produced?  The  problem  is  of  just 
the  same  sort  in  the  Protozoa  that  it  is  in  ourselves.  In  such 
a  case  as  Difflugia  the  answer  which  suggests  itself  takes 
something  of  this  form :  the  offspring  produce  the  same  sort 
of  shell  that  the  parents  did,  because  they  are  made  of  the 
same  sort  of  material.  This  answer  is  indeed  little  but  a 
form,  but  it  is  a  form  into  which  a  more  complete  answer 
will  have  to  fit. 

Before  inquiring  further  as  to  just  how  closely  the  off- 
spring do  produce  the  same  characteristics  as  the  parents, 
let  us  look  at  another  example  or  two,  showing  in  a  still 
more  marked  way  the  nature  of  inheritance.  In  some  of  the 
more  complex  Protozoa,  such  as  the  hypotrichous  infusoria, 
we  find  that  the  body  bears  a  great  number  of  organs  of 
definite  form  and  number,  arranged  in  a  precise  manner. 
Observe  in  this  Stylonychia  (Figure  13),  the  numerous  leg- 
like or  fin-like  appendages,  for  creeping  or  swimming.  When 
the  animal  reproduces,  it  divides  cross-wise,  and  if  it  merely 
divided  without  any  rearrangement  of  parts,  you  can  see 
that  the  two  progeny  would  be  most  unlike  the  parents.  As 
a  matter  of  fact,  during  reproduction  all  these  organs  dis- 
appear; they  are  apparently  gradually  absorbed  into  the 
body.  Then  on  each  half  of  the  body  there  appears  (even 
before  the  disappearance  of  the  old  organs)  a  new  group 
of  minute  projections  (Figure  13,  B),  all  close  together; 
not  arranged  at  all  as  were  the  appendages  of  the  parent, 
and  not  showing  the  differences  of  size  and  structure  that 
were  found  in  the  parent.  These  small  projections  now 
proceed  to  change  place,  scatter  themselves,  and  take  up 
positions  corresponding  to  those  of  the  appendages  of  the 


Heredity  m  Protozoa 


43 


parent  (Figure  13,  C)  ;  at  the  same  time  they  transform  in 
various  ways,  till  they  have  produced  anew  a  set  of  organs 
like  those  of  the  parent,  of  the  same  number,  form,  size  and 
arrangement. 


Figure  13.  Reproduction  in  Stylonychia,  after  Wallengren  (1901). 
A,  Parent  before  reproduction.  B,  Appearance  of  the  two  groups  of 
small  projections  that  are  to  form  the  appendages  of  the  two  offspring. 
C,  Division  is  occurring;  the  two  groups  of  embryonic  appendages  are 
scattering  out  to  take  up  their  final  positions.  The  old  appendages 
have  not  yet  disappeared. 

All  this  illustrates  the  general  nature  of  the  problem  of 
heredity  in  both  lower  and  higher  organisms;  the  features 
which  the  offspring  are  said  to  inherit  from  the  parents  they 
really  produce  anew  for  themselves,  and  the  problem  is  as  to 
why  and  to  what  extent  they  produce  the  same  things  that 
the  parents  did. 

There  are  certain  exceptions  to  this  general  rule  that  the 
progeny  have  to  produce  anew  what  they  inherit;  a  few 


44  Life  and  Death,  Heredity  and  Evolution 

things  are  directly  handed  on  from  parent  to  offspring. 
These  few  things  are  of  great  importance,  for  it  is  these 
that  in  some  way  provide  the  foundation  for  the  new  pro- 
duction of  the  other  structures.  In  the  Protozoa  these  are 
as  follows :  ( 1 )  The  halves  of  the  nuclei ;  in  cases  where 
there  exist  separate  active  nuclei  and  reserve  nuclei  (macro- 
and  micro-nuclei),  a  half  of  each  of  these  is  passed  on  to 
each  of  the  progeny  (see  Figure  7).  (2)  Secondly,  a  half  of 
the  general  protoplasmic  mass,  or  cytoplasm,  goes  to  each 
of  the  progeny.  As  we  have  seen,  the  particular  organs 
borne  by  this  mass  usually  are  not  handed  on  bodily,  but 
first  disappear  and  are  then  produced  anew  by  the  offspring. 
Some  few  definite  organs  are  in  particular  cases  passed  on 
bodily,  but  even  in  these  cases  at  least  half  of  them  must 
be  produced  anew, — else  of  course  they  would  occur  only 
in  half  as  great  number  in  each  of  the  two  offspring.  All  the 
precision  of  the  process  of  division  and  handing  on  is  seen 
in  the  nucleus  (see  Figure  49,  page  182),  so  that  it  seems 
probable  that  its  different  parts,  each  so  accurately  divided, 
have  special  and  diverse  functions  to  perform  in  the 
production  of  the  new  organs  of  the  progeny.  Just  what 
part  is  played  by  each  of  these  things  that  are  directly 
handed  on  from  parent  to  offspring,  in  producing  the  final 
characters  of  the  progeny,  is  one  of  the  chief  questions  of 
heredity. 

When  we  have  gotten  to  our  present  point  in  the  examina- 
tion of  reproduction  and  heredity  in  these  creatures,  we  shall 
not  make  the  mistake  of  some  of  the  earlier  writers,  as  to 
the  inheritance  of  acquired  characters  in  the  Protozoa.  By 
acquired  characters  we  mean  characters  that  an  individual 
did  not  inherit  from  its  parents,  but  which  were  produced 
by  special  conditions  during  its  life.  Since  the  parents  mere- 
ly divide  and  become  the  offspring,  it  was  set  forth  as  a 


Heredity  of  Acquired  Characters  45 

matter  of  course  that  in  the  Protozoa  the  offspring  would 
receive  the  acquired  characters  as  well  as  the  other  char- 
acters of  the  parent.  But  as  we  have  seen,  it  does  not 
receive  even  the  other  characters  of  the  parent ;  but  produces 
them  anew.  Is  there  any  reason  why  the  offspring  should 


Figure  14.  Inheritance  from  mutilated  parents,  in  Difflugia  corona. 
The  three  parents,  A,  B  and  C,  have  had  their  shells  and  spines  broken. 
The  offspring  of  each  is  seen  helow  it,  With  normal,  unmutilated  shells 
and  spines.  The  mutilations  of  the  parents  are  not  inherited  by  the 
offspring.  (From  observations  by  the  author.) 

produce  anew  characters  that  the  parent  has  acquired  merely 
accidentally,  owing  to  special  circumstances?  Let  us  look 
at  a  few  of  these.  There  is  no  place  so  favorable  for  getting 
in  a  simple  way  a  clear  idea  of  the  problems  and  difficulties 
involved  in  the  "inheritance  of  acquired  characters"  as  in 
the  Protozoa.  To  many  persons  who  have  not  examined  the 


46  Life  and  Death,  Heredity  and  Evolution 

details  it  seems  extraordinary  that  anyone  should  doubt  that 
acquired  characters  are  inherited. 

Let  us  examine  first  the  simplest  possible  case,  of  an  ac- 
quired injury  or  mutilation.  Suppose  that  in  Difflugia  one 
or  more  of  the  spines  is  broken  off,  or  a  hole  is  broken  in  the 
shell  (Figure  14).  Is  there  any  reason  why  when  this 
animal  reproduces,  the  progeny  should  have  a  corresponding 
broken  spine  or  broken  shell?  The  offspring  produces  anew 


Figure  15.  Reproduction  in  an  infusorian  (Paramecium)  in  which 
the  anterior  end  has  been  cut  off.  At  the  first  division  the  posterior 
offspring  is  quite  normal,  and  by  the  third  division  even  the  anterior 
offspring  has  regained  its  normal  anterior  tip.  Only  the  divisions  of 
the  mutilated  individual  are  represented,  as  all  others  produce  normal 
offspring.  After  Jennings,  1908. 

its  own  shell  and  spines;  what  sort  it  shall  produce  under 
given  outer  conditions  depends  on  the  nature  of  its  proto- 
plasm and  nucleus.  These  are  not  altered  by  the  breaking 
of  the  shell  or  spine  of  the  parent,  and  as  a  matter  of  fact 
we  find  that  the  offspring  produce  entire  shells  and  spines, 
just  as  their  parents  did.  The  injuries  acquired  by  the 
parent  are  not  inherited.  Again,  look  at  an  infusorian  with 
its  anterior  tip  cut  off  (Figure  15).  Is  there  any  reason 
why  the  offspring  produced  from  the  hinder  half  should  have 
its  anterior  tip  cut  off?  As  a  matter  of  fact,  it  has  not. 


Heredity  of  Acquired  Characters 


Figure  16.  Reproduction  for  several  generations  of  a  Paramecium 
bearing  a  large  projection  on  its  aboral  surface.  At  each  division  only 
one  individual  receives  the  projection;  all  other  offspring  and  descend- 
ants are  normal.  After  Jennings,  1908. 


Figure  17.  Reproduction  in  a  Paramecium  bearing  a  small  abnormal 
projection  near  the  posterior  end.  All  offspring  but  one  are  normal 
from  the  beginning,  and  even  the  abnormal  individual  becomes  normal 
after  two  divisions.  After  Jennings,  1908. 


48  Life  and  Death,  Heredity  and  Evolution 

Or  observe  an  infusorian  in  which  a  new  structure — a  pro- 
jection for  example — has  been  produced  through  some  ac- 
cident (Figure  16).  Will  this  thing  be  inherited  at  repro- 
duction? It  will  naturally  be  carried  on  mechanically  by  a 
single  one  of  the  individuals  at  reproduction,  but  the  rest 
do  not  produce  it.  After  a  thousand  new  individuals  have 
been  produced,  one  of  them  may  still  have  the  new  structure, 
but  no  more  than  one.  As  a  matter  of  fact,  such  a  new 
structure  is  usually  gotten  rid  of  completely  in  the  making 
over  that  accompanies  reproduction  (Figure  17).  I  have 


Figure  18.  Reproduction  in  a  deformed  individual  of  Paramecium. 
After  the  second  division  all  descendants  are  normally  formed.  After 
Jennings,  1908. 

examined  a  large  number  of  such  cases  in  the  infusorian 
Paramecium;  .the  acquired  peculiarities  are  not  inherited 
(see  Figures  15  to  18). 

So  there  is  no  simple  direct  inheritance  of  acquired  char- 
acters in  the  Protozoa,  any  more  than  there  is  in  the  higher 
organisms.  The  progeny  have  to  produce  the  characters 
that  they  get,  just  as  the  parents  did, — and  they  usually 
produce  what  the  parent  produced  when  it  developed, — not 
what  the  parent  may  accidentally  happen  to  have  when  it 
divides.  The  progeny  start  where  the  parent  did,  as  a 
rule. 

Of  course  if  we  could  get  the  fundamental  constitution  of 


Heredity  in  Protozoa  49 

the  organism  to  change — say  the  chemical  nature  of  its 
nucleus  and  cytoplasm — we  might  expect  it  to  develop  in  a 
new  way,  and  produce  new  structures ;  and  as  this  would  be 
repeated  in  later  generations,  we  should  have  the  new  char- 
acters inherited.  But  this  is  a  deep  and  difficult  matter; 
we  shall  take  it  up  later.  What  we  wish  to  bring  out  now 
is  the  fact  that  the  mere  existence  of  an  acquired  character 
in  the  parent  presents  no  reason  for  expecting  the  progeny 
to  produce  that  character  anew,  and  that  as  a  matter  of 
fact  they  do  not ;  such  acquired  characters  are  not  inherited, 
either  in  Protozoa  or  in  more  complex  organisms. 

We  have  seen  thus  that  the  progeny  do  not  receive  their 
organs  ready  made  from  the  parent;  that  on  the  contrary 
they  start  from  just  the  same  condition  the  parents  did — 
an  undifferentiated  condition  without  organs,  on  the  whole 
— and  produce  their  characteristics  anew.  The  ground  for 
their  producing  the  same  characteristics  as  the  parents 
lies  precisely  in  this  fact,  that  they  start  just  as  the  parents 
did. 

Having  thus  examined  the  groundwork  for  inheritance,  we 
wish  to  look  at  the  matter  more  minutely.  Is  it  true  that 
the  offspring  start  just  where  the  parents  did,  and  that  they 
produce  just  what  the  parents  did?  Or  is  there  a  gradual 
change  as  generations  pass,  so  that  evolution  occurs? 

If  the  progeny  begin  just  where  the  parents  did  and  de- 
velop in  the  same  way,  then  if  we  begin  with  a  single  parent 
and  obtain  from  it  great  numbers  of  progeny  in  successive 
generations,  we  shall  find  that  they  are  the  same  at  the 
end  as  they  were  at  the  beginning;  and  that  all  are  alike 
(save  in  so  far  environmental  differences  have  made  diver- 
sities). There  will  be  no  opportunity  for  some  to  be  pre- 
served because  they  are  better  fitted  to  live,  while  others 
die  because  ill  fitted.  Is  this  the  situation,  or  do  inherited 
variations  come  on  as  generations  pass? 


50  Life  and  Death,  Heredity  and  Evolution 

This  problem  of  the  origin  of  inherited  variations,  and  of 
the  nature  and  grounds  of  evolution,  meets  us  in  these 
animals  in  a  peculiarly  simple  form.  For  in  their  repro- 
duction from  a  single  parent  it  is  not  complicated  by  the 
continual  mixing  of  diverse  lines  of  descent,  which  enormous- 
ly confuses  the  matter  in  higher  organisms.  In  creatures 
in  which  reproduction  is  always  from  two  parents,  the  de- 
scent of  generations  takes  the  form  of  a  network,  such  as 
illustrated  at  B  in  Figure  5.  If  we  attempt  to  trace  back 
the  ancestry  of  any  one  of  the  individuals  indicated  by  the 
lines  at  the  right,  we  find  that  it  is  a  mixture  of  many  lines 
of  descent,  with  diverse  hereditary  characters ;  in  any  given 
past  generation  many  ancestors  of  the  present  stock  appear. 
It  becomes  extremely  difficult  or  impossible  to  predict  what 
hereditary  characters  it  should  show,  or  whence  it  has  de- 
rived those  that  do  appear;  and  it  is  hardly  possible  to 
distinguish  an  actually  new  character  from  one  resulting 
from  the  mixture  of  earlier  stocks.  In  the  Protozoa  while 
descent  from  a  single  parent  is  in  progress,  the  passage  of 
generations  takes  the  form  indicated  at  A  in  Figure  5. 
There  is  no  mixing  of  diverse  lines,  and  in  any  past  genera- 
tion there  is  but  one  ancestor  for  the  existing  stock ;  descent 
can  be  traced  backward  in  a  single  line.  On  account  of  this 
relatively  simple  state  of  affairs,  the  origin  of  variations  and 
the  course  of  evolution  has  been  much  studied  in  the  Proto- 
zoa. We  will  illustrate  the  conditions  found  by  means  of  a 
series  of  figures  of  Difflugia  corona  (Figures  19-21). 

When  we  examine  a  single  species,  be  it  a  bacterium,  a 
rhizopod  (as  Difflugia  corona)  or  a  ciliate  infusorian  (as 
Paramecium  aurelia),  we  find  a  great  diversity  in  the  in- 
dividuals, in  their  form,  their  structure  (Figure  19),  and 
their  physiology.  Part  of  this  diversity  is,  in  some  of  these 
creatures,  due  to  the  different  conditions  under  which  they 
are  living.  But  if  we  bring  them  all  into  the  same  conditions 


Variation  in  Protozoa 


51 


Figure  19.  Difflugia  corona;  collection  of  individuals  to  show  the 
variations  in  size  and  form;  in  number,  length  and  shape  of  the  spines, 
and  the  like.  All  drawn  to  the  same  scale.  (The  numbers  are  the 
designations  of  the  families  to  which  the  individuals  belonged.)  After 
Jennings,  1916. 


52  Life  and  Death,  Heredity  and  Evolution 


FIG.  20a. 


Inheritance  in  Difflugia 


FIG. 


Figure  20.  Difflugia  corona.  Parent  and  immediate  offspring  in  18 
diverse  families,  all  drawn  to  the  same  scale,  to  show  the  variation  and 
the  inheritance  of  the  parental  characteristics  by  the  progeny.  In  each 
pair  the  parent  is  above,  its  offspring  below,  the  two  connected  by  a 
line.  If  each  family  is  bred  for  many  generations,  it  continues  to 
remain  true  to  the  type  shown.  (The  numbers  are  the  designations  of 
the  families  to  which  the  individuals  belonged.)  After  Jennings,  1916. 


54s  Life  and  Death,  Heredity  and  Evolution 

and  allow  them  to  reproduce,  we  find  that  the  offspring  re- 
peat in  large  measure  the  peculiarities  of  their  parents 
(Figure  20).  That  is,  the  particular  characteristics  of  the 
parents  are  inherited, — quite  independently  of  diversity  in 
conditions.  If  we  allow  each  of  the  diverse  parents  to  re- 
produce for  generation  after  generation,  we  find  that  in  each 
case  the  peculiarities  of  the  original  stock  are  retained 
(Figure  21).  That  is,  each  single  species,  such  as  this  one 
of  Difflugia  corona,  consists  of  a  large  number  of  hered- 
itarily diverse  strains  or  families;  of  strains  remaining  di- 
verse for  generation  after  generation. 

This  is  one  of  the  facts  of  capital  importance  in  the 
biology  of  these  organisms ;  something  that  has  to  be  kept 
continuously  in  mind  in  all  attempts  to  work  with  them  or 
to  understand  them.  It  forms  the  key  and  explanation  for 
many  remarkable  phenomena  in  their  lives.  We  shall  there- 
fore look  at  the  concrete  facts  for  a  number  of  typical  cases, 
and  examine  their  results  in  relation  to  heredity  and  varia- 
tion. 

In  Difflugia  corona  the  number  of  hereditarily  diverse 
strains  that  have  been  found  is  indefinitely  great.  They 
differ  in  size  and  form,  in  the  number  of  spines,  in  the  length 
of  the  spines,  in  the  number  of  teeth  surrounding  the  mouth. 
Different  strains  have  hereditarily  different  combinations  of 
these  characters ;  some  have  large  shells  with  few  spines ; 
others  have  large  shells  with  many  spines,  and  so  on  for 
other  combinations.  " 

Besides  these  marked  structural  peculiarities,  the  strains 
of  Difflugia  differ  in  many  other  ways  not  apparent  to  the 
eye.  Some  of  the  strains  multiply  rapidly,  others  slowly. 
Some  are  very  hardy  and  easily  cultivated  in  the  laboratory ; 
others  are  delicate,  dying  out  under  artificial  conditions. 
Some  are  very  active,  others  quiet.  Some  are  adapted  to 


Inheritance  in  Difftugia 


55 


Figure  21.  Difflugia  corona;  portions  of  four  families,  to  show  the 
inheritance  of  the  diverse  combinations  of  characters.  All  individuals 
of  any  column  are  descendants  of  the  one  at  the  top.  The  num- 
bers at  the  top  are  the  designations  of  the  different  families.  Ob- 
serve that  in  family  198  all  are  small,  but  with  numerous  spines;  in 
197,  they  are  larger,  but  with  few  spines.  In  324-  the  individuals  are 
large,  with  large  spines;  in  323  they  are  of  about  the  same  size,  but 
with  small  spines.  The  families  remain  true  to  such  peculiarities,  no 
matter  how  long  they  are  bred.  After  Jennings,  1916. 


56  Life  and  Death,  Heredity  and  Evolution 

one  set  of  conditions,  others  to  other  conditions.  The  exist- 
ence of  these  strains  presents  an  enormous  diversity  within 
the  species ;  anything  that  we  learn  about  one  strain  cannot 
be  transferred  directly  to  another.  A  similar  condition  of 
affairs  has  since  been  found  to  occur  in  other  Rhizopods. 
Root,1  (1918)  shows  that  many  diverse  stocks  exist  in 
Centropyxis  aculeata;  and  Hegner,  2  (1918)  shows  the  same 
to  be  true  in  Arcella  dentata. 

In  many  other  Protozoa  most  of  the  differences  between 
the  stocks  are  in  respect  to  the  characters  not  readily  de- 
tectible  by  the  eye.  In  Paramecium  caudatum,  or  Para- 
mecium  aurelia,  for  example,  the  visible  differences  between 
the  strains  are  mainly  in  respect  to  size,  and  since  the  size 
is  changed  greatly  during  growth,  it  requires  thorough  study 
to  detect  the  differences  of  strain.  But  such  study  shows 
that  strains  of  different  size  do  exist  (Figure  22).  Within 
each  strain  there  is  great  variation  of  size  among  the  dif- 
ferent individuals,  owing  to  differences  of  growth  and  of 
environment.  But  each  strain  or  family  has  its  own  char- 
acteristic average  size.  If  we  pick  out  any  individual  of  a 
given  strain  and  allow  it  to  produce  many  offspring,  we  shall 
find  that  their  average  size  will  correspond  to  that  of  the 
family  from  which  they  came;  an  individual  from  a  larger 
strain  will  produce  larger  progeny ;  one  from  a  smaller  strain 
will  produce  smaller  progeny.  Thus  a  "population"  of 
Paramecium  as  we  find  it  in  nature  is  made  up  in  the  way 
shown  in  Figure  22.  There  are  many  strains,  diverse  in 
mean  size,  but  each  with  many  individuals  of  diverse  size. 

In  Paramecium,  as  in  Difflugia,  these  strains  differ  in  other 
respects  also.  I  found  that  some  multiply  rapidly,  others 
slowly;  that  some  conjugate  frequently,  others  rarely. 

'Root,  F.  M.,  Genetics,  March,  1918. 

2  Hegner,  R.  W.,  Proceedings  of  National  Academy,  September,  1918. 


Diverse  Strains  in  Protozoa 


57 


Figure  22.  Eight  diverse  families  of  Paramecium,  showing  varia- 
tions. Each  row  represents  a  single  family,  showing  the  maximum, 
minimum  and  intermediate  sizes  of  individuals  of  the  given  family. 
The  differences  in  size  within  the  family  are  due  to  differences  in 
growth  and  environment.  The  differences  in  average  size  between  the 
families  are  hereditary. 

The  numbers  show  the  lengths  in  microns.  The  mean  length  for  the 
entire  set  together  is  given  by  the  perpendicular  line  at  155  microns. 
The  mean  size  for  each  family  is  that  of  the  individual  above  which  is 
placed  a  +  sign.  After  Jennings,  1909. 


Hutchison  (1913)  found  that  they  differ  in  their  resistance 
to  heat, — some  strains  standing  a  higher  temperature  than 
others;  the  same  thing  was  observed  by  Jollos  (1913). 


58  Life  and  Death,  Heredity  and  Evolution 

Jollos  observed  also  that  some  strains  are  more  resistant 
to  poisons  than  others.  Hance  (1917)  found  that  a  cer- 
tain race  of  Paramecium  caudatum  has  a  tendency  to  pro- 
duce one  to  three  extra  contractile  vacuoles,  so  that  its 
members  may  have  three  to  five  of  these,  in  place  of  the  two 
found  in  most  races.  Powers  and  Mitchell  (1910)  found 
a  race  of  Paramecium  that  had  several  micronuclei,  in  place 
of  the  single  one,  or  the  pair,  commonly  found. 

All  together,  it  is  clear  that  the  different  races  of  Par- 
amecium present  the  greatest  diversities  in  all  sorts  of 
structural,  and  particularly  physiological,  characters, — so 
that  from  our  knowledge  of  the  biology  of  one  race  we  cannot 
be  certain  as  to  that  of  the  others. 

A  similar  condition  of  affairs  is  known  to  exist  in  many 
species  of  bacteria.  Diverse  families  exist,  differing  in  their 
nutritive  peculiarities,  in  their  resistance  to  chemical  agents, 
in  their  virulence  as  producers  of  disease;  in  the  way  they 
grow  on  artificial  media,  and  the  like. 

The  condition  of  affairs  has  been  found  to  exist  in  all 
the  Protista  that  have  been  thoroughly  studied  from  this 
point  of  view.  It  is  probable  that  it  will  be  found  in  all 
species ;  certainly  it  would  be  of  much  interest  to  examine 
thoroughly  any  species  that  seems  to  be  uniform,  in  order 
to  discover  if  there  is  such  a  thing  as  a  species  that  does 
not  consist  of  hereditarily  diverse  stocks. 

The  same  condition  of  things  is  likewise  found  in  higher 
organisms.  It  is  worth  while  to  recall  the  name  and  work 
of  the  man  who  first  recognized  that  the  so-called  species  of 
animals  and  plants  are  really  made  up  of  a  great  number  of 
hereditarily  diverse  stocks,  which  remain  quite  distinct  so 
long  as  they  are  reproduced  without  crossing ;  for  this  work 
was  done  long  ago,  and,  like  the  work  of  Mendel,  remained 
for  many  years  quite  without  influence  on  the  world.  In 


Diverse  Strains  m  Higher  Organisms  59 

recent  years  it  has  been  rediscovered,  and  some  believe  that 
it  is  almost  worthy  to  rank  with  the  great  work  of  Mendel, 
whose  obscure  fate  it  long  shared.  The  discoverer,  Alexis 
Jordan,  was,  like  Mendel,  a  devout  Catholic ;  and  was  guided 
in  his  experimentation  (unlike  Mendel,  apparently)  by  his 
theological  beliefs.  He  did  not  believe  in  the  variability  of 
organisms,  as  taught  by  many  prevailing  doctrines,  but 
maintained  that  the  differences  within  a  species  that  were 
commonly  cited  as  variations  were  in  reality  permanent  dif- 
ferences between  races.  So  as  early  as  1854  he  undertook 
the  culture  in  his  garden  of  certain  common  plants,  notably 
Draba  verna,  the  common  little  weed  called  whitlow  grass. 
In  ten  years  he  was  able  to  show  that  this  contained  ten 
diverse  races ;  after  twenty  years'  culture  he  announced  that 
he  had  now  found  53  races ;  and  after  culture  for  thirty 
years  he  could  show  that  there  were  200  permanently  diverse 
stocks  of  Draba  verna.  He  discovered  the  same  thing  to 
be  true  for  a  number  of  other  plants,  and  maintained  there- 
fore that  his  faith  Jiad  been  verified;  the  differences  found 
within  a  species  were  not  variations  in  the  sense  of  actual 
changes  which  occurred,  but  merely  permanent  diversities, 
which  Jordan  believed  had  existed  ever  since  the  organisms 
were  created;  instead  of  variations  that  occurred,  there  was 
multiplicity  that  existed.1 

Now  whatever  we  may  think  of  Jordan's  line  of  argument, 
the  facts  which  he  set  forth  have  been  confirmed  in  recent 
years  for  a  great  number  of  organisms.  And  his  direct 
conclusion  from  those  facts  likewise  stands  fast.  The  dif- 
ferences that  we  observe  among  the  members  of  a  species  are 
in  the  overwhelming  majority  of  cases  not  "variations"  in 
the  sense  of  being  due  to  recent  actual  changes  in  the  hered- 

irThe  facts  as  to  the  work  of  Jordan  are  taken  mainly  from  Lotsy, 
1916  and  1916  a. 


60  Life  and  Death,  Heredity  and  Evolution 

itary  constitution.  On  the  contrary  they  are  merely  lasting 
hereditary  diversities  between  stocks  whose  origin  has  not 
been  observed.  The  situation  is  very  greatly  confused  when 
these  diverse  stocks  continually  intercross,  as  happens  in  so 
many  higher  organisms,  but  it  is  not  thereby  essentially 
changed.  ^ 

The  situation  in  these  lower  organisms  is  very  much  what 
it  would  be  in  man  if  in  man  new  individuals  were  regularly 
produced  by  the  division  of  those  already  existing.  We 
have  reason  to  believe  that  this  practically  does  occur  in  the 
case  of  identical  twins;  they  are  produced  by  the  division 
of  a  single  egg,  which  if  it  had  not  divided  would  have  pro- 
duced but  one  individual  (see  Newman,  1917).  If  this 
occurred  in  man  regularly  and  frequently,  as  it  does  in  most 
Protozoa,  we  should  find  that  the  human  population  .con- 
tained great  numbers  of  individuals  as  precisely  alike  as  are 
identical  twins.  Each  of  us  would  meet  his  precise  counter- 
part at  every  turn.  All  these  closely  similar  individuals  of 
one  type,  taken  together,  would  correspond  to  a  single  one 
of  the  stocks  or  races  of  the  Protozoa.  And  as  in  the 
Protozoa,  there  would  exist  great  numbers  of  such  diverse 
stocks ;  in  man  as  many  as  there  now  exist  hereditarily  dif- 
ferent individuals.  That  is,  each  person  present  in  this 
room  would  represent  a  diverse  stock  or  race;  for  each 
person  has  a  constitution  hereditarily  diverse  from  every 
other  (save  in  the  case  of  identical  twins). 

Now  this  fact  that  a  species  consists  of  a  great  number 
of  hereditarily  diverse  stocks  or  races,  often  differing  in 
only  minute  particulars,  throws  a  most  unexpected,  and  to 
many  persons  unwelcome,  flood  of  light  on  many  supposed 
studies  of  evolution,  and  particularly  on  the  effects  of 
selection  in  the  breeding  of  organisms, — giving  to  such 
studies  a  significance  quite  diverse  from  that  which  they 


Selection  Among  Diverse  Strains  61 

were  supposed  to  have.  A  great  school  of  biologists,  the 
immediate  followers  of  Darwin,  forming  the  so  called  English 
biometrical  school,  set  themselves  the  problem  of  measuring 
variation,  inheritance,  the  effects  of  selection,  and  from 
these  the  rate  of  evolution.  In  so  doing  they  assumed  these 
existing  differences  as  variations,  and  based  their  calcula- 
tions upon  these ;  they  found  of  course  a  high  degree  of  in- 
heritance from  diverse  parents,  and  they  found  that  by 
selection  rapid  progress  could  be  made  in  changing  the 
species.  But  if  we  examine  a  species  made  up  of  a  lot  of 
hereditarily  diverse  strains  (for  example,  Diiflugia  corona), 
it  is  evidently  easy  by  selection  of  a  particular  character  to 
obtain  a  set  of  animals  that  differ  from  the  average  in  that 
respect;  one  merely  picks  out  representatives  of  the  races 
that  have  the  character  for  which  we  are  selecting.  Thus, 
in  Difflugia  corona,  if  one  desires  to  increase  the  average 
numbers  of  spines,  he  will  pick  out  parents  with  many 
spines.  These  belong  to  races  in  which  a  large  number 
of  spines  is  hereditary,  so  that  after  selection  the  progeny 
produced  will  have  a  higher  number  of  spines  than  was  the 
average  for  the  species  before  selection.  By  continuing 
the  process  of  selection,  we  gradually  exclude  more  and  more 
completely  the  races  with  few  spines,  and  so  by  selection  we 
make  steady  progress  in  increasing  the  number  of  spines  in 
this  animal.  Similar  results  follow  from  selecting  for  any 
other  inherited  character;  and  in  any  species  composed  of 
diverse  races,  the  same  sort  of  results  are  reached. 

Thus  in  a  population  of  Paramecium  we  can  easily  obtain 
by  selection  all  sorts  of  apparent  hereditary  alterations  in 
size;  in  nutritive  peculiarities;  in  resistance  to  heat  or  to 
chemicals ;  in  reproduction ;  in  all  sorts  of  characters.  All 
these  and  many  other  results  have  been  produced  again  and 
again,  in  this  and  in  other  species.  But  what  we  really  do 


62  Life  and  Death,  Heredity  and  Evolution 

is  to  pick  out  races  that  already  have  these  peculiarities. 
This  is  unquestionably  the  explanation  of  the  effects  of 
selection  in  by  far  the  greater  number  of  experiments  or 
observations  where  it  is  found  to  have  an  effect. 

Such  results  were  long  interpreted  as  showing  actual  steps 
in  evolution;  by  selection  changes  in  hereditary  characters 
were  thought  to  be  produced,  and  new  hereditary  characters 
obtained.  On  the  basis  of  such  interpretations  the  rate  of 
change  through  selection,  the  rate  of  evolution,  was 
measured. 

But  what  really  occurs  in  all  such  cases  is  a  gradual 
picking  out  of  existing  races  with  certain  characteristics 
(in  our  example,  with  numerous  spines),  and  discarding  the 
rest.  So  far  as  the  selection  is  based  on  these  differences 
between  preexisting  stocks,  no  evolutionary  change  has  been 
produced  or  measured. 

But  can  selection  do  nothing  more  than  this?  Are  there 
no  actual  changes  in  hereditary  characters?  Are  these 
diverse  races  really  permanent  in  their  hereditary  char- 
acters? Or  do  changes  gradually  occur  even  within  such 
stocks?  How  do  these  diverse  races  happen  to  exist?  Can 
several  diverse  races  be  produced  from  a  single  one? 

On  this  question  a  great  deal  of  work  has  been  done  in 
the  unicellular  forms ;  we  shall  examine  the  results. 

One  of  the  animals  most  studied  from  this  point  of  view  is 
Paramecium.  If  we  take  a  wild  set  of  these  animals,  it  is 
easy,  as  we  have  seen,  to  bring  about  differences  by  selection, 
for  all  we  have  to  do  is  to  pick  out  such  as  we  please  of  the 
different  races  that  exist.  But  suppose  we  take  but  a  single 
race;  suppose  we  begin  with  a  single  individual  and  get  our 
entire  population  from  this  one  parent.  Shall  we  then  be 
able  by  selection  to  bring  about  hereditary  differences?  In 
other  words,  do  any  hereditary  changes  occur  in  such  a 
single  race?  This  question  is  evidently  the  fundamental  one 


Effect  of  Selection  63 

for  evolution,  for  if  no  such  changes  occur,  there  would 
appear  to  be  no  such  thing  as  evolution. 

I  made  long  continued  attempts  to  change  the  size  in 
such  a  race  of  Paramecium,  by  selecting  on  the  one  hand 
large  individuals ;  on  the  other  hand  small  ones  (Jennings, 
1908).  No  effect  was  produced.  Large  parents  and  small 
parents,  if  they  belonged  to  the  same  race,  produced  progeny 
of  the  same  size.  Ackert  (1916)  has  recently  repeated  these 
experiments,  with  the  same  results. 

I  tried  also  to  produce  from  a  single  race  stocks  differing 
in  their  rate  of  reproduction,  but  these  attempts,  like  those 
to  change  the  size,  met  with  no  success.  Each  race  seemed 
permanent  in  its  size  and  rate  of  reproduction. 

Jollos  (1913)  attempted  by  selection  in  Paramecium  to 
obtain  stocks  that  differed  in  their  resistance  to  heat  and 
to  chemicals.  He  found  this  very  easy  when  he  began  with 
a  wild  population  containing  many  diverse  stocks;  all  he 
had  to  do  was  to  isolate  the  differing  stocks.  But  when  he 
began  with  a  single  stock,  he  found  that  he  could  not  by 
selection  get  stocks  diverse  in  their  resistance.  Throughout 
the  ordinary  multiplication,  all  the  individuals,  like  identical 
twins,  remained  just  alike. 

Similar  results  have  followed  many  other  attempts  to 
change  by  selection  the  inherited  characteristics  of  a  single 
stock.  Barber  (1908)  made  long  continued  attempts  to 
change  in  this  way  the  characteristics  of  pure  stocks  of 
bacteria  and  of  yeasts.  He  found  a  very  few  cases  of  sud- 
den mutation,  such  as  occur  with  similar  rarity  in  higher 
organisms ;  of  these  we  shall  speak  later.  But,  as  a  rule,  any 
differences  found  within  a  single  race  were  not  inherited; 
the  races  were  permanent.  Wolf  (1909)  made  repeated  and 
long  continued  attempts  to  change  through  selection  the 
colors  in  pure  races  of  the  red  bacterium,  Bacillus  pro- 
digiosus ;  but  he  could  produce  no  change  in  this  way. 


64  Life  and  Death,  Heredity  and  Evolution 

Thus  in  general,  until  very  recently  at  least,  the  experience 
of  investigators  has  been  such  as  to  confirm  what  was  said 
many  years  ago  by  that  greatest  of  investigators  of  the 
Protozoa,  Emile  Maupas.  In  1888  Maupas,  after  long  con- 
tinued study  of  Protozoa,  said: 

"In  long  and  numerous  experiments  on  fifteen  to  twenty 
species,  I  have  never  observed  anything  which  permits  belief 
in  the  existence  of  morphological  and  physiological  differ- 
ences between,  not  merely  the  products  of  a  given  fission, 
but  even  among  all  those  which  have  descended  from  such  a 
fission  by  regular  and  continuous  generations."  2 

The  same  sort  of  results  have  been  reached  from  the 
study  of  higher  organisms  when  they  reproduce  without  mix- 
ture,— the  progeny  arising  from  a  single  parent  instead  of 
two.  The  most  famous  work  of  this  sort  is  the  study  of 
self-fertilizing  beans,  made  by  Johannsen  (1903).  Diverse 
races  existed,  but  in  seven  years  of  selection  no  effect  was 
produced  on  the  characters  studied,  so  long  as  the  selection 
occurred  within  the  limits  of  a  single  race.  Lashley  (1915) 
studied  Hydra  in  the  same  way ;  no  effect  was  produced  from 
selection  continued  for  many  generations.  Ewing  (1916) 
attempted  to  modify  by  selection  plant  lice  multiplying 
parthenogenetically.  The  work  extended  over  eighty-seven 
generations,  and  many  different  characters  were  investigated 
for  longer  or  shorter  periods.  In  no  case  could  he  change 
the  hereditary  characteristics  by  selection.  Agar  (1914) 
studied  in  a  similar  manner  certain  of  the  lower  crustacea 
multiplying  by  parthenogenesis,  and  reached  the  same  re- 
sults. A  great  number  of  investigations  could  be  cited,  all 
ending  in  the  same  way.  The  organisms  studied  contained 
many  diverse  races.  But  when  a  single  race  or  "line"  is 
studied,  not  mixing  with  other  races,  the  differences  between 

'Maupas,  1888,  p.  176. 


Constancy  of  Strains  65 

individuals  were  not  inherited,  and  long  continued  selection 
was  without  effect.  This  sort  of  study  has  come  to  be 
known  as  the  "pure  line"  work,  and  the  general  result  of 
it  all  has  been  that  "selection  has  no  effect  within  a  pure 
line." 

Such  results  have  profoundly  modified  the  theories  of 
heredity  and  of  evolution.  The  Danish  investigator  Jo- 
hannsen  (1913)  has  based  on  them  and  similar  results  a 
general  system  or  doctrine  of  heredity.  According  to  Jo- 
hannsen's  views,  the  hereditary  constitution  of  a  given  organ- 
ism is  a  perfectly  definite  thing,  not  subject  to  gradual  or 
indefinite  fluctuation.  This  constant  hereditary  constitution 
he  calls  the  genotype.  The  genotype  is  comparable  to  a 
definite  chemical  compound.  It  may  become  altered  some- 
times, as  a  definite  chemical  compound  may  by  certain  reac- 
tions be  transformed  into  another  and  diverse  compound; 
such  complete  transformations  are  called  mutations ;  they 
are  extremely  rare.  The  genotype  is  not  subject  to  slight 
and  gradual  alterations,  any  more  than  is  the  nature  of  the 
chemical  compound  NaCl.  When  diverse  genotypes  are 
mingled,  as  in  reproduction  from  two  parents,  the  results 
give  us  what  is  called  Mendelian  inheritance.  We  cannot 
take  up  the  details  of  this  here,  but  the  same  principles  hold. 
Selection  in  such  cases,  according  to  this  view,  merely  brings 
about  varied  combinations  of  things  already  existing;  the 
nature  of  its  effects  is  therefore  the  same  as  when  it  is  ap- 
plied to  organisms  descending  from  but  a  single  parent. 
This  view  of  the  matter  may  perhaps  be  said  to  have  dom- 
inated recent  work  in  heredity. 

This  constancy  of  races  in  organisms  descending  from  a 
single  parent,  and  the  application,  which  appears  unavoid- 
able, of  the  same  point  of  view  to  the  cases  where  there  are 
two  parents,  presents  very  great  difficulties  for  the  theory 


66  Life  and  Death,  Heredity  and  Evolution 

of  evolution.  It  could  indeed  be  plausibly  maintained  that 
all  these  results  are  opposed  to  the  theory  of  evolution ;  that 
the  logical  conclusion  to  be  drawn  is  that  which  was  main- 
tained by  Jordan,  the  originator  of  this  sort  of  work — 
namely,  that  there  is  no  variation;  no  change  from  genera- 
tion to  generation ;  no  evolution. 

Is  that  indeed  the  conclusion  to  which  we  are  driven?  Is 
that  the  upshot  of  the  modern  attempts  to  study  evolution 
experimentally;  to  see  evolution  occur?  We  shall  take  up 
this  question  in  our  next  chapter. 


Ill 


Results  of  Intense  and  Long  Continued  Study  of  Changes 
in  a  Stock.  Inherited  Variations  in  the  Pure  Race.  Visible 
Evolution. 

T  N  our  last  chapter  I  tried  to  picture  the  first  results  of 
•*•  the  attempts  to  study  evolution  experimentally — to  ac- 
tually see  evolution  occurring — to  see  variations  take  place 
and  to  see  their  inheritance. 

These  first  results  were: 

That  any  kind  of  organism  is  really  composed  of  a  great 
number  of  diverse  stocks  or  races,  whose  differences  are 
hereditary,  lasting  from  generation  to  generation; 

That  the  supposed  effect  of  selection  in  modifying  organ- 
isms consists  in  isolating  certain  pre-existing  races,  having 
the  characteristics  that  one  is  selecting;  or  in  making, 
through  biparental  inheritance,  new  combinations  of  charac- 
ters that  already  exist ; 

That  in  general,  the  apparent  variations  in  organisms 
are  not  real  changes  in  their  hereditary  constitution,  but 
are  merely  these  static  diversities  persisting  from  genera- 
tion to  generation ; 

That  when  one  takes  a  single  one  of  these  races  and  tries 
to  discover  in  it  hereditary  variations,  or  to  modify  it  by 
selection,  he  finds  it  extraordinarily  constant,  and  his  efforts 
are  without  result.  That  seemingly  the  only  variations 
which  appear  are  sudden  large  mutations;  that  gradual 
alterations  do  not  show  themselves. 
67 


68  Life  and  Death,  Heredity  and  Evolution 

I  wish  to  emphasize  that  this  is  a  picture  of  the  present 
situation  of  affairs;  that  it  gives  the  prevailing  theory  of 
these  matters.  If  you  will  read  the  address  of  President 
Pearl  of  the  American  Society  of  Naturalists,  published  in 
the  American  Naturalist  (Feb.  1917),  you  will  find  that  this 
is  the  theory  which  he  presents.  I  have  no  doubt  that  if 
there  are  any  experimental  workers  in  heredity  in  this  audi- 
ence, this  is  the  theory  which  they  maintain. 

Now,  as  I  remarked  at  the  end  of  the  last  chapter,  this 
situation  of  affairs  presents  great  difficulties  for  the  theory 
of  evolution, — leading  indeed  logically  to  the  conclusion 
that  evolution  does  not  occur.  And  even  when  we  add  to 
these  results  the  observed  cases  of  sudden  change  of  charac- 
ters, which  are  called  mutations,  it  becomes  extremely  diffi- 
cult to  see  how  evolution  can  occur.  For  most  if  not  all  of 
these  mutations,  as  is  well  known,  consist  in  defects  and 
losses ;  and  it  is  difficult  to  believe  that  evolution  has  occurred 
by  repeated  losses, — although  attempts  have  been  made  to 
maintain  even  that  paradoxical  theory  (Bateson,  1914; 
Davenport,  1916). 

Moreover,  there  are  certain  facts  about  organisms  which 
it  seems  impossible  to  explain  by  the  appearance  of  sudden 
extensive  mutations.  In  the  organisms  that  we  have  been 
describing  we  find  that  the  hereditary  differences  between 
the  races  are  as  minute  as  can  possibly  be  detected  by  the 
most  refined  methods ;  they  run  down  to  the  very  limits  of 
visibility  with  the  microscope.  Such  are  the  differences  be- 
tween the  diverse  races  of  Paramecium  (Figure  22);  such 
those  between  the  races  of  Difflugia  (Figures  19  to  21).  It 
is  clear  that  such  differences  cannot  have  been  produced  by 
"saltations" — by  mutations  of  large  extent.  And  the  same 
condition  of  affairs  is  found  in  higher  organisms ;  the  differ- 
ences between  Jordan's  200  races  of  Draba  verna  were  so 


Inheritance  of  Variations  69 

slight  that  it  took  years  to  detect  them  with  certainty.  If 
such  minutely  differing  races  have  been  produced  from  a 
single  one,  the  steps  in  the  change  have  been  at  least  as 
minute  as  these  differences.  If  evolution  really  occurs,  why 
should  we  not  see  these  minute  changes? 

Again,  the  existence  of  complex  adaptive  structures,  such 
as  the  eye  or  the  ear,  presents  difficulties  for  the  theory 
of  origin  by  extensive  mutations  perhaps  fully  as  great  as 
does  the  existence  of  races  differing  only  by  minute  grada^ 
tions.  The  difficulty  here  is  not  so  readily  presented  in  a 
simple  way,  but  to  me  it  appears  that  thorough  analysis 
would  reveal  it  as  an  insuperable  one. 

Furthermore,  paleontologists  maintain,  with  practical 
unanimity,  that  the  study  of  extinct  organisms  shows  that 
the  change  in  the  characteristics  of  animals  of  a  given  stock, 
as  we  pass  from  one  geological  period  to  another,  has  not 
been  by  leaps,  but  by  gradual  alterations.  The  study  of 
paleontology  is  the  most  direct  study  possible  of  past  evolu- 
tion ;  we  cannot  neglect  its  conclusions. 

On  the  whole  then  it  is  difficult  to  rest  content  with  the 
results  of  what  we  may  call  this  first  examination  of  the 
diversities  in  such  organisms.  Shall  we  yield  to  the  argu- 
ment of  Jordan,  that  evolution  is  not  occurring?  Or  shall 
we  rather  proceed  to  more  refined  studies ;  to  what  might  be 
called  investigations  in  the  second  degree?  In  our  first 
studies  have  we  not  possibly  been  overwhelmed  and  halted 
by  the  great  discovery  that  most  of  what  we  had  thought 
were  real  variations  and  real  effects  of  selection  were  de- 
ceptive,— were  mere  consequences  of  the  existence  of  heredi- 
tarily diverse  races, — so  that  we  have  stopped  before  the 
end  ?  Shall  we  not  next  merely  accept  all  this  that  has  been 
learned  in  the  last  fifteen  years  as  a  background,  take  a 
new  hold;  select  the  most  favorable  organism  possible;  avoid 


70  Life  and  Death,  Heredity  and  Evolution 

all  sources  of  confusion  met  in  the  earlier  studies,  limit  our- 
selves to  one  single  race,  pursue  its  history  with  more  minute 
and  unwearying  steadiness,  for  longer  periods  than  has 
before  been  done, — to  such  a  degree  that  we  may  properly 
call  our  studies  investigations  in  the  second  degree, — as 
compared  with  the  earlier  ones  ? 

This  is  what  I  decided  four  years  ago  to  attempt ;  with  a 
number  of  associates  to  set  on  foot  such  "second  degree" 
investigations.  They  have  now  been  carried  far  enough  to 
show  results.  The  main  difficulties  in  the  previous  work 
along  these  lines  have  been  the  following: 

(1)  In   these   simple   organisms    it   is    difficult   to    find 
definite  distinctive  characters,  such  as  color  of  eyes  or  hair, 
that  are  inherited.     In  Paramecium,  for  example,  all  the  in- 
dividuals are  very  similar,  the  diversities  being  mainly  slight 
differences  in  size  or  shape;  or  in  indefinite  physiological 
traits.      Such    characters    are   not   favorable   for  work   in 
heredity  because  they  are  hard  to  distinguish;  yet  in  prac- 
tically all  the  earlier  work  such  characters  were  employed, — 
for  the  good  reason  that  these  seemed  the  only  characters 
available. 

(2)  Further,  the  characters  studied  have  been  such  as 
were  continually  changed  by  growth  during  the  life  of  the 
animals ;  and  likewise  greatly  modified  by  changes  in  environ- 
mental conditions. 

The  first  thing  to  do  therefore  was  to  find  if  possible  an 
organism  in  which  these  difficulties  did  not  exist.  From  this 
point  of  view  an  ideal  animal  was  found  in  Difflugia  corona, 
— the  creature  that  I  have  already  employed  in  these  lec- 
tures to  illustrate  a  number  of  points.  This  animal  is  an 
amceba  with  a  shell,  and  the  shell  presents  a  number  of 
definite  characters  that  can  be  easily  counted  and  measured 
(Figure  23).  These  characters  are:  the  number  of  spines; 


Inheritance  of  Variations 


71 


Figure  23.  Difflugia  corona,  to  show  the  characters  studied  in  the 
work  on  inheritance.  A.  Side  view.  B.  Oral  view,  showing  the  mouth 
and  the  teeth  surrounding  it.  After  Jennings,  1916. 

the  length  of  the  spines ;  the  number  of  the  teeth  about  the 
mouth  (Figure  23,  B)  ;  and  the  size  of  the  shell,  as  meas- 
ured by  its  diameter.  All  these  characters  are  formed  when 
the  individual  is  produced  (Figures  11  and  12),  and  are  not 
subject  to  change  by  growth,  nor  are  they  altered  by  changes 
in  the  environment  during  the  life  of  the  individual.  No 
more  favorable  combination  of  characters  for  the  study  of 
variation  and  heredity  could  possibly  be  found. 

Difflugia  is  extremely  fastidious  as  to  just  how  it  shall 
live  and  what  sort  of  food  it  shall  be  furnished,  so  that  it 
is  not  easy  to  keep  pedigreed  stock  for  generation  after 
generation,  as  is  necessary  for  all  work  in  heredity.  When 
these  difficulties  are  overcome,  we  follow  for  long  periods  the 
history  of  a  given  race.  As  we  have  seen,  when  we  compare 
different  races,  the  peculiarities  of  the  parents  reappear  in 
the  offspring  in  a  high  degree  (Figures  19  to  21).  But  di- 
versities do  arise  even  within  a  single  race.  Parents  with 
many  spines  have  as  a  rule  progeny  with  many  spines,  but 


72  Life  and  Death,  Heredity  and  Evolution 

often  the  number  is  not  the  same  in  the  progeny,  and  the  suc- 
cessive progeny  of  the  same  parent  may  have  different  num- 
bers of  spines.  As  we  pass  from  parent  to  offspring,  similar 
variations  arise  as  to  length  of  spines,  number  of  teeth,  and 
size  of  the  shell.  These  facts  are  illustrated  in  Figures  19 
and  20. 

Now  the  question  of  interest  is,  whether  these  differences 
within  the  single  race  (all  derived  by  fission  from  a  single 
parent)  show  any  tendency  to  be  inherited.  When  a  single 
parent  produces  one  offspring  with  few  spines  and  another 
with  many  spines,  does  the  former  tend  then  to  produce  a 
set  of  progeny  with  few  spines,  the  latter  a  set  with  many? 
If  so,  we  have  the  beginning  of  the  origin  of  two  races  from 
one.  Or  will  there  be  mere  chance  variations, — with  no  tend- 
ency on  the  part  of  the  later  descendant  to  reproduce  its 
parent's  peculiarities  ? 

Study  shows  that  there  is  certainly  no  complete  or  even 
very  marked  tendency  for  the  progeny  within  a  race  to 
reproduce  the  diverse  peculiarities  of  the  parents.  If  a 
parent  has  many  spines,  some  of  its  offspring  have  many, 
some  few ;  if  the  parent  has  few  spines,  some  of  its  offspring 
have  many  spines,  some  few ;  and  so  of  all  the  other  charac- 
ters (see  Figures  20  and  21).  To  get  any  results  whose 
meaning  is  clear,  we  have  to  resort  to  averages,  and  to 
mathematical  measures  of  correspondence,  for  very  large 
numbers  of  parents  and  progeny  belonging  to  a  single  race. 
What  we  have  to  do  is  to  determine  whether,  within  a  single 
stock,  on  the  average  and  in  the  long  run,  parents  with  many 
spines  produce  offspring  with  a  greater  number  of  spines 
than  do  parents  with  few  spines. 

When  we  do  this  we  find  in  Difflugia  indications  that  there 
is  some  correspondence  between  parents  and  progeny.  In 
some  cases  the  indications  are  very  slight;  in  others  more 


Inheritance  of  Variations  73 

marked.  Measuring  by  the  coefficient  of  correlation,  we  find 
that  when  the  parent  differs  from  the  average,  the  progeny 
tend  to  inherit  somewhere  from  one-tenth  to  three-tenths  of 
this  peculiarity.  That  is,  the  race  shows  a  slight  tendency 
to  break  up  into  several  races,  hereditarily  distinct.  (For 
details  see  Jennings,  1916.) 

For  the  investigator  who  has  searched  in  vain  for  years 
to  find  in  uniparental  reproduction  any  tendency  for  a  single 
race  to  evolve  into  several,  such  faint  indications  are  excit- 
ing ;  here  we  begin  to  get  hold  of  the  beginnings  of  evolution. 
Most  of  the  grounds  on  which  we  believe  that  evolution 
occurs  are  inferential ;  we  believe  that  it  must  have  occurred, 
— in  order  to  account  for  the  diversities  that  we  find  now 
existing.  But  can  we  actually  see  it  occur? 

To  carry  our  work  farther,  we  begin  to  exercise  selection 
within  the  single  family.  On  the  one  hand  we  select  all  the 
long-spined  individuals  and  place  them  together;  on  the 
other  hand  we  select  all  the  short-spined  ones  and  place  them 
together.  In  the  long-spined  group  we  continue  to  save  for 
generation  after  generation  only  the  individuals  that  are 
long-spined;  in  the  short-spined  group  only  the  offspring 
with  short  spines.  In  the  same  way  we  select  other  sets  for 
numerous  spines  and  for  few  spines ;  for  large  shells  and 
for  small  shells ;  for  many  teeth  and  for  fewer  teeth. 

And  now  as  we  keep  this  up  for  generation  after  genera- 
tion we  find  that  the  correspondence  between  parent  and 
progeny  becomes  more  and  more  marked.  We  find  that 
our  single  family  is  breaking  up  into  many  different  groups, 
which  differ  from  one  another  hereditarily.  We  get  finally 
what  appear  to  be  two  diverse  races, — one  with  long  spines, 
the  other  with  short  spines  (Figures  24,  M  and  F), — 
the  difference  continuing  for  generation  after  generation. 
A  third  set  (L)  has  constantly  large  shells,  while  others 


74  Life  and  Death,  Heredity  and  Evolution 


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Inheritance  of  Variations 


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76  Life  and  Death,  Heredity  and  Evolution 

(M  and  N)  consistently  produce  small  shells.  We  also  get 
stocks  hereditarily  different  for  numbers  of  spines ;  and  for 
numbers  of  teeth.  Our  single  stock,  derived  by  fission  from 
a  single  parent,  has  gradually  diversified  itself  into  many 
stocks  that  are  hereditarily  different.  If  this  is  what  we 
me*an  by  evolution,  we  have  here  seen  evolution  occur. 

In  Figure  24  we  see  a  number  of  the  hereditarily  diverse 
stocks  that  arose  by  fission  from  a  single  parent,  in  one  of 
my  experiments.  It  will  be  worth  while  to  summarize  the 
main  facts  as  to  the  appearance  of  hereditary  variations  in 
this  animal. 

(1)  Hereditary  variations  arose  in  some  few  cases  by 
rather  large   steps,  which  might  be   called   mutations,   or 
saltations. 

(2)  But  the  immense  majority  of  the  hereditary  varia- 
tions were  minute  gradations.     Variations,  are  as  continu- 
ous as  can  be  detected. 

(3)  Hereditary  variations  occurred  in  many  different 
ways,  on  many  diverse  characters :  the  number  of  spines ; 
the  length  of  the  spines ;  the  size  of  the  body ;  the  num- 
ber of  teeth.    There  was  no  single  line  of  variation  that  was 
followed  exclusively,  or  by  the  great  majority  of  cases. 

(4)  Any  set  of  characters  gave  rise  to  variations  in- 
dependently of  the  other  characters.     Thus  many  diverse 
combinations  of  characters  arose;  large  animals  with  long 
spines ;  small  animals  with  long  spines ;  large  animals  with 
short  spines;  short  animals  with  short  spines,  and  so  on 
for  other  sorts  of  combinations. 

(5)  The  hereditary  variations  which  arose  were  of  just 
such  a  nature  as  to  produce  from  a  single  strain  the  heredi- 
tarily different  strains  that  are  found  in  nature. 

I  judge  that  if  the  intermediate  strains  were  killed  off,  the 
two  most  diverse  strains  found  in  nature  might  well  be 


Inheritance  of  Variations  77 

classed  as  different  species,- — although  the  question  of  what 
constitutes  a  species  must  be  left  to  the  judgment  or  fancy 
of  the  individual. 

Since  the  work  on  Difflugia  was  done,  the  same  methods  of 
work  have  been  applied  in  our  laboratory  to  two  of  its  rela- 
tives, Centropyxis  aculeata  and  Arcella  dentata.  In  the 
former,  Root  (1918)  found  considerable  evidence  that  cer- 
tain variations  within  the  single  stock  were  inherited,  al- 
though the  work  was  not  carried  so  far  as  in  Difflugia.  In 
Arcella,  Hegner  (1918)  found  that  heritably  diverse  stocks 
could  be  isolated  by  selection  from  a  single  stock  multiplying 
by  fission. 

All  together  then,  our  "second  degree"  study  of  the  mat- 
ter has  been  rewarded  by  the  discovery  that  for  these  animals 
at  least  the  situation  that  I  sketched  in  my  last  lecture  is 
not  final.  In  these  animals  we  do  find  the  diverse  races  present 
under  natural  conditions,  just  as  in  other  organisms  (see 
Figures  20  and  21);  and  by  mere  selection  among  these 
diverse  races  we  can  get  all  sorts  of  apparent  changes — 
which  are  not  real  changes;  which  are  not  evolution.  But 
when  we  take  a  single  race  and  devote  all  our  attention  to 
that  alone  for  years,  then  we  find  that  real  changes  do 
occur;  that  the  race  differentiates  into  many  races  in  the 
way  I  have  described ;  that  evolution  visibly  does  occur. 

Now  I  told  you  that  the  other  theory  was  the  prevailing 
one;  so  much  is  this  the  case  that  some  of  my  readers 
will  not  accept  unreservedly  these  cases  as  actual  changes 
in  hereditary  constitution;  as  actual  steps  in  evolution;  on 
the  contrary  they  are  trying  to  devise  various  possible 
schemes  by  which  it  could  be  made  to  seem  that  even  here 
in  Difflugia  we  are  getting  nothing  but  new  combinations  of 
what  was  before  present.  Many  such  schemes  have  been 
devised  for  explaining  apparent  effects  of  selection  in  higher 


78  Life  and  Death,  Heredity  and  Evolution 

organisms.  None  of  them  can  be  applied  directly  to  Dif- 
flugia,  since  here  we  have  uniparental  reproduction,  and  most 
of  these  schemes  depend  upon  the  mixing  of  two  stocks.  But 
other  schemes  can  be  devised,  which  might  apply  to  Dif- 
flugia;  we  shall  mention  some  of  these.  At  present  I  wish 
to  ask  your  patience  for  a  few  moments  for  a  closer  analysis 
of  just  what  has  happened  in  such  a  case  as  this.  Such  an 
analysis  will  bring  out  the  main  questions  and  difficulties 
that  can  be  raised. 

What  then  is  it  that  has  actually  occurred  in  such  a  case  ? 
We  began  with  a  single  individual;  it  consisted  of  a  shell 
filled  with  a  mass  of  protoplasm,  containing  one  or  more 
nuclear  bodies.  This  mass  had  behaved  in  such  a  way  as  to 
produce  a  shell  of  definite  size,  form,  number  of  spines,  and 
the  like.  We  found  that  when  this  mass  of  protoplasm  gives 
off  one-half  of  itself  to  the  outside  of  the  old  shell,  this  half 
is  made  up, — chemically  or  otherwise, — in  the  same  way 
as  was  the  original  parent  mass;  for  it  does  just  the  same 
things  that  the  parent  mass  did.  That  is,  it  produces  a 
shell  essentiailly  like  that  of  the  parent, — of  a  similar  size, 
shape,  number  of  spines,  and  the  like  (see  Figure  12).  This 
is  particularly  striking  when  we  compare  this  individual  with 
others  of  different  race,  or  of  different  species,  as  in  Figure 
20;  it  is  extraordinary  to  see  these  tiny  masses  of  proto- 
plasm, each  conducting  itself  in  a  manner  different  from  any 
other;  each  holding  true  to  type.  There  must  be  very  defi- 
nite, and  at  the  same  time  very  delicate,  chemical  differences 
between  them. 

But  as  we  follow  for  a  long  time  our  original  individual 
and  its  progeny,  we  find  that  the  chemical  nature  of  the 
protoplasm  very  gradually  changes  as  divisions  occur,  for 
the  behavior  begins  to  slightly  change.  Although  all  under 
the  same  conditions,  some  of  the  masses  commence  to  produce 


Inheritance  of  Variations  79 

longer  spines ;  others  shorter ;  others  more  numerous  spines ; 
others  fewer  (Figure  24).  Different  masses  change  in  dif- 
ferent ways ;  the  number  of  kinds  of  diversity  that  we  get  is 
large;  apparently  indefinitely  large.  The  protoplasm  cer- 
tainly gradually  becomes  diversified  as  it  continues  to  exist 
and  increase. 

A  number  of  important  questions  at  once  arise.  What 
part  of  the  protoplasm  is  it  that  thus  changes?  Is  it  the 
cytoplasm,  or  the  nucleus,  or  is  it  both?  And  how  does 
the  change  occur?  Through  irregularities  in  the  division 
of  certain  substances  or  parts?  Or  through  chemical  or 
physical  changes  produced  by  the  environment, — by  changes 
in  temperature,  chemical  composition  of  the  water,  by  food, 
or  the  like?  And  why  do  we  find  these  changes  to  occur  in 
Difflugia  when  we  could  not  find  them  in  Paramecium;  when 
in  almost  all  the  other  organisms  studied  in  this  way  such 
changes  have  not  been  discovered? 

All  these  questions  are  bound  up  together.  We  shall 
perhaps  deal  best  with  them  by  taking  up  the  last  question 
first.  Why  do  we  find  such  a  difference  in  this  respect  be- 
tween Difflugia  and  other  organisms, — for  example,  Para- 
mecium? 

As  was  set  forth  earlier,  Difflugia  was  selected  for  study 
precisely  because  it  was  much  more  favorable  for  such  work 
than  Paramecium  or  most  other  organisms  investigated.  In 
Difflugia  there  are  many  well  defined  distinctive  characters, 
which  are  not  modified  by  growth,  nor  changed  by  the  con- 
ditions under  which  the  animals  live.  In  Paramecium  and 
most  other  forms  studied,  on  the  other  hand,  the  characters 
are  continually  changing  through  growth  and  environmental 
action.  Such  changes  are  well  known  not  to  be  inherited. 
This  makes  a  tremendous  difference  as  to  accomplishing 
anything  by  selecting  any  particular  character.  Thus  in 


80  Life  and  Death,  Heredity  and  Evolution 

Figure  22,  showing  the  diverse  races  of  Paramecium,  we  see 
that  in  any  single  race  there  are  individuals  of  many  different 
sizes.  But  almost  all  these  differences  are  matters  of  age, 
nutrition,  and  the  like.  So  when  we  select  and  separate 
large  and  small  individuals,  we  are  likely  to  get  merely  well- 
grown,  well-fed  individuals  in  one  set;  young,  ill-nourished 
ones  in  the  other.  Even  if  there  are  arising  really  hereditary 
differences  in  size,  we  cannot  distinguish  these  from  the  much 
more  numerous  transitory  changes,  so  that  our  process  of 
selection  may  be  rendered  quite  without  hereditary  effect. 
Similar  difficulties  beset  any  attempt  to  select  for  other 
characters  that  are  dependent  on  growth  and  present  en- 
vironmental conditions. 

It  appears  possible  therefore  that  the  difference  between 
the  results  of  selection  in  Difflugia  and  in  other  organisms  is 
due  to  these  facts ;  that  there  is  no  real  difference  as  to  the 
sort  of  thing  that  happens,  but  only  as  to  whether  one  can 
detect  the  hereditary  changes  that  actually  occur.  Are  we 
to  believe  that  the  hereditary  constitution  of  parent  and 
progeny  is  actually  identical  in  these  other  forms?  Or  shall 
we  find  changes  in  it,  if  we  study  with  sufficient  minuteness? 

Now  on  this  point  we  have  a  certain  amount  of  evidence, 
based  again  on  what  I  have  ventured  to  call  our  "second 
degree"  investigation  of  these  matters.  If  we  watch  the 
divisions  of  Paramecium  or  of  any  of  its  relatives,  we  find 
that  the  two  individuals  produced  by  the  division  of  one 
do  not  always  behave  exactly  alike ;  after  they  have  grown 
to  adult  size,  one  of  them  often  divides  before  the  other 
does  (see  Figure  25).  Are  such  differences  due  to  some 
change  in  the  fundamental  and  hereditary  constitution,  or 
only  to  some  slight  difference  in  nutrition  or  the  like  ?  Here 
was  an  opportunity  for  minute  study  of  the  matter;  it  was 
undertaken  in  our  laboratory  by  Middleton  (1915).  He 


Inheritance  of  Variations 


81 


investigated  from  this  point  of  view  the  infusorian  Stylony- 
chia.  Beginning  with  a  single  individual,  he  selected  on  the 
one  hand  the  offspring  that  divided  first ;  on  the  other  those 
that  divided  last.  Continuing  to  select  for  rapid  fission  rate 
in  one  line,  for  slow  fission  rate  in  another,  and  keeping  this 
up  for  hundreds  of  generations,  he  found  after  many  gen- 
erations that  there  were  real  inherited  differences.  Two  sets 


Figure  25.  The  infusorian  Stylonychia;  diagram  to  show  the  differ- 
ences in  time  of  fission  in  offspring  of  the  same  parent,  with  the  method 
of  selection  for  rapid  fission  and  slow  fission.  Constructed  on  the  baws 
of  a  figure  of  Middleton,  1915. 

A,  the  parent  divides  into  two  offspring  (B),  one  of  which  divides 
into  two  while  the  other  still  remains  undivided  (C).  Similar  condi- 
tions appear  in  D  and  E.  Thus  in  JJ,  the  large  individual  at  the  right 
is  the  result  of  but  two  divisions  from  A,  while  the  small  individuals 
to  the  left  are  the  result  of  four. 

were  produced  from  among  the  descendants  of  a  single  par- 
ent,— one  set  that  divided  more  rapidly  than  the  other. 
The  difference  persisted  for  long  after  selection  had  stopped. 

Thus  it  was  shown  that  in  this  case  hereditary  diversities 
are  arising,  even  with  respect  to  a  character  so  readily 
modified  by  the  environment  as  is  the  fission  rate. 

This,  so  far  as  it  goes,  of  course  tends  to  raise  the  pre- 
sumption that  other  characters  of  such  organisms  will  be 
found  to  show  similar  hereditary  changes  when  studied  with 


82  Life  and  Death,  Heredity  and  Evolution 

sufficient  thoroughness, — so  that  there  would  be  no  real  dif- 
ference in  this  respect  between  them  and  Difflugia.  I  should 
like  to  say,  however,  that  from  experience  with  Difflugia 
and  other  organisms,  and  from  the  work  of  other  investi- 
gators, I  am  personally  convinced  that  there  is  a^  difference 
between  organisms  as  to  the  frequency  with  which  hereditary 
variations  occur.  They  occur  on  the  whole  relatively  fre- 
quently in  Difflugia.  In  many  other  organisms  the  germinal 
material  is  apparently  so  protected,  and  so  precisely  divided 
at  reproduction,  that  such  changes  are  rare. 

And  this  brings  us  to  the  question  as  to  just  what  in  the 
organism  it  is  that  is  altered  when  the  hereditary  characters 
change.  A  number  of  possibilities  are  open  here.  Hegner  * 
(1919)  discovered  that  in  Arcella  the  hereditary  size  varies 
with  the  number  of  nuclei  or  the  amount  of  chromatin  pres- 
ent, and  that  these  change  at  times  as  a  result  of  irregulari- 
ties in  division.  The  number  of  spines  further  was  found  to 
be  related  to  the  size;  larger  individuals  have  more  spines 
than  smaller  ones.  Hence  hereditary  diversities  in  the  num- 
ber of  spines  also  were  brought  about  by  alterations  in  the 
number  of  nuclei  or  volume  of  chromatin.  Some  of  the  heredi- 
tary changes  in  the  characters  of  Difflugia  may  have  been 
brought  about  in  the  same  way,  but  it  is  clear  that  most  of 
them  were  not.  For  as  we  have  seen,  some  of  the  new  lines 
produced  were  small  with  large  spines,  some  large  with  large 
spines, — the  different  ^characters  being  independent  in  their 
hereditary  diversities,  so  as  to  give  stocks  with  different  com- 
binations of  characters.  These  cannot  be  accounted  for  by 
quantitative  alterations  in  the  amount  of  the  nuclear  material. 
Another  possibility  lies  in  certain  peculiarities  of  the  nu- 
cleus in  such  animals  as  Difflugia.  There  is  in  addition  to  one 
or  more  very  sharply  defined  nuclei  a  cloud-like  mass\>f  nu- 
'Hegner,  R.  W.,  Proceedings  of  National  Academy,  January,  1919. 


Nature  of  Heritable  Variations  83 

clear  material  spread  through  the  cytoplasm ;  this  is  known 
as  a  chromidium.  Unfortunately  these  things  are  not  yet 
thoroughly  known  for  Difflugia  itself,  but  in  some  apparently 
close  relatives,  such  as  Arcella  (Figure  26),  they  have 
been  much  studied.  In  the  division  of  these  organisms  the 
nuclei  divide  with  the  same  minute  precision  that  is  evident 
in  higher  creatures.  But  the  chromidial  masses  merely  sep- 
arate loosely  into  halves,  with  no  indication  of  precision.  So 
possibly  the  offspring  may  get  different  parts  of  the 
chromidium  in  different  cases,  and  it  has  been  suggested  that 


Figure  26.  Arcella  vulgaris,  to  show  the  two  nuclei  (N),  and  the 
chromidium  (C),  or  loose  cloud  of  nuclear  material.  After  Hertwig. 

the  differences  that  arise  in  the  hereditary  characters  are 
due  to  this  inexactness  of  division.  But  this  is  all  specula- 
tion as  yet,  without  much  foundation  of  probability. 

But  in  any  case  it  appears  to  me  that  these  details  do  not 
affect  the  main  fact,  which  is  that  in  these  organisms  gradual 
inherited  variations  are  occurring,  so  that  in  the  course  of 
time  many  hereditarily  diverse  families  arise  from  one.  In 
other  words,  if  we  study  these  organisms  with  sufficient  mi- 
nuteness and  perseverance,  we  see  evolution  occurring. 

We  are  now  in  position  to  sum  up  the  facts  as  to  heredity 
and  variation  in  these  animals  when  they  are  reproducing 
from  a  single  parent.  Any  species  consists  of  a  great  num- 


84  Life  and  Death,  Heredity  and  Evolution 

ber  of  hereditarily  diverse  families  or  races,  whose  charac- 
teristics show  a  high  degree  of  permanence  from  generation 
to  generation.  The  offspring  inherit  in  a  high  degree  the 
characteristics  of  the  parent.  But  this  inheritance  is  not 
through  an  actual  handing  on  of  the  parent's  characteris- 
tics ;  on  the  contrary  the  offspring  have  to  produce  anew 
the  same  kind  of  characters  that  the  parent  had.  For  this 
reason  any  peculiarities  acquired  by  the  parent  during  its 
life  time  are  not  inherited. 

Inheritance  is  very  exact,  but  when  we  study  a  family  for 
many  generations,  we  find  that  it  is  not  absolutely  precise, 
for  minute  hereditary  variations  gradually  appear,  and  the 
single  race  separates  into  many  hereditarily  diverse  races. 
The  process  of  evolution  becomes  visible. 


IV 


Can  We  Experimentally  Change  the  Hereditary  Charac- 
ters? Heredity  of  Environmental  Effects.  Heredity  and 
Variation  m  Bacteria  and  Similar  Organisms. 

\  \  7"E  have  seen  that  in  Difflugia  hereditary  variations 
*  V  arise  even  when  the  organisms  are  all  kept  as  nearly 
as  possible  under  the  same  conditions.  Thus  from  a  single 
strain,  all  derived  by  fission  from  one  ancestor,  many  strains 
arise,  diverse  in  their  hereditary  characters. 

Can  such  changes  be  brought  about  by  the  action  of 
special  conditions  in  the  environment?  Can  we  experimen- 
tally produce  such  hereditary  changes?  Have  some  of  the 
diverse  strains  existing  in  nature  been  produced  by  action 
of  the  environment?  We  saw  in  our  introductory  lecture 
that  structural  characters  produced  in  the  body  of  Protozoa 
during  the  lifetime  are  not  inherited  any  more  directly  than 
are  such  characters  in  higher  organisms.  To  be  inherited, 
the  acquired  structures  must  be  produced  anew  by  the  off- 
spring, and  for  most  acquired  characters  this  does  not 
occur;  the  offspring  are  produced  in  the  same  state  that 
the  parents  were.  But  there  still  remains  the  question 
whether  the  organisms  cannot  be  so  altered  that  in  the 
succeeding  generations  characters  will  be  produced  that  are 
different  from  those  produced  in  the  earlier  generations. 

1.    Bacteria 

Most  of  the  significant  work  bearing  on  this  question  has 
been  done  on  the  bacteria,  and  on  other  organisms  that 
85 


86  Life  and  Death,  Heredity  and  Evolution 

have  to  do  with  the  production  of  disease.  In  bacteria 
the  knowledge  of  variation  and  heredity  has  passed  through 
the  same  series  of  stages  that  we  noted  in  other  organisms. 
At  first  there  seemed  to  be  a  mere  chaos  of  diverse  forms, 
with  no  constancy  or  order;  any  kind  of  bacterium  seemed 
producible  from  any  other,  or  from  other  organic  or  inor- 
ganic sources.  Then  came  a  period  of  thorough  study,  with 
development  of  precise  technical  methods.  It  was  discov- 
ered that  there  are  a  very  great  number  of  kinds  of  bacteria, 
but  that  each  remains  true  to  its  characters ;  each  is  pro- 
duced only  by  pre-existing  individuals  of  the  same  race. 
The  differences  between  the  races  are  often  minute,  mere 
matters  of  a  slight  diversity  in  the  chemical  processes,  in 
the  kind  of  sugar  that  is  fermented  by  the  particular  race, 
or  the  like.  But  each  race  remained  true  to  its  type,  even 
in  these  minute  physiological  details.  This  stage  of  knowl- 
edge is  the  same  as  that  on  which  is  based  the  theory  of 
the  constancy  of  genotypes  in  all  sorts  of  organisms, — a 
theory  that  we  sketched  in  Lecture  2.  The  constancy  of 
the  races  of  bacteria  has  been  set  forth  as  one  of  the  facts 
opposed  to  the  theory  that  organisms  are  undergoing  evolu- 
tionary changes. 

But  in  recent  years  a  still  more  intensive  study  has 
brought  to  light,  in  this  group  as  in  others,  the  actual 
occurrence  of  hereditary  changes,  the  production,  from  a 
given  race,  of  other  races  whose  hereditary  characters  are 
diverse  from  those  of  the  parent  race.  In  the  bacteria,  more 
than  in  any  other  group  of  organisms,  something  has  been 
learned  of  the  conditions  which  bring  about  these  changes, 
though  knowledge  on  this  point  is  still  scanty. 

The  bacteria  present  extreme  difficulties  for  the  critical 
study  of  heredity  and  variation,  owing  to  their  minuteness. 
To  be  certain  of  what  the  results  mean  it  is  necessary  to 


Inheritance  in  Bacteria  87 

work  with  races  all  members  of  which  are  derived  from  a 
single  original  individual.  It  was  long  impossible  to  ful- 
fill this  requirement;  the  so-called  pure  cultures  of  bacteria 
were  derived  from  a  large  number  of  individuals.  There 
might  be  slight  racial  differences  between  these  original  indi- 
viduals, the  different  strains  being  adapted  to  different  con- 
ditions. Then  under  given  conditions  one  strain  multiplied 
until  the  entire  population  seemed  to  take  on  its  characteris- 
tics, the  other  strains  remaining  without  activity  or  mul- 
tiplication. But  on  a  change  of  conditions  this  prevalent 
strain  ceased  its  multiplication,  while  some  other  strain 
became  active,  multiplying  until  the  population  showed  the 
characteristics  of  this  second  strain.  It  appeared  as  if  the 
changed  environment  had  altered  the  hereditary  characteris- 
tics of  the  organisms,  but  this  appearance  would  be  illusory. 
It  seems  probable  that  such  impurity  of  the  original  stock 
accounts  for  some  of  the  apparent  transformations  that 
have  been  described. 

But  in  recent  years  a  number  of  methods  have  been  de- 
vised for  isolating  a  single  bacterium,  so  that  an  entire 
stock  can  be  derived  from  this.1  From  such  pure 'races 
dependable  results  can  be  obtained. 

One  of  the  first  to  isolate  pure  stocks  was  Barber  (1907)  ; 
he  worked  both  with  bacteria  and  with  yeasts.  Barber  did 
not  attempt  to  modify  the  organisms,  but  merely  to  deter- 
mine whether  the  variations  in  size  and  form  often  observed 
are  inherited.  From  a  pure  race  he  picked  out  large  indi- 
viduals, long  individuals,  individuals  of  peculiar  form,  and 

1  Apparently  the  simplest  and  most  effective  method  is  that  of 
mixing  a  very  little  of  the  fluid  containing  bacteria  with  a  large  quan- 
tity of  India  ink,  then  producing  a  thin  layer  of  this  ink  between  two 
cover  glasses.  Single  bacteria,  owing  to  the  fact  that  they  are  sur- 
rounded by  a  cloud  of  gelatinous  material,  are  visible  as  minute  clear 
specks  in  the  dark  ink.  A  cover  glass  preparation  containing  but  a 
single  individual  is  taken  as  the  beginning  of  a  culture.  This  is  known 
as  Burri's  method. 


88  Life  and  Death,  Heredity  and  Evolution 

the  like,  and  determined  whether  their  descendants  inherited 
their  peculiarities. 

Barber  discovered  the  same  thing  that  others  have  found 
in  other  organisms;  in  the  very  great  majority^of  cases  such 
peculiarities  within  a  race  are  not  inherited.  Great  num- 
bers of  indivduals  selected  for  certain  peculiarities  gave 
offspring  of  the  usual  types.  Nevertheless  a  few  heritable 
variations  were  discovered.  In  Bacillus  coli,  140  individuals 
that  were  longer  than  usual  were  isolated.  All  but  one 
gave  descendants  of  the  usual  size,  but  this  one  gave  a  race 
having  bodies  longer  than  usual.  The  race  was  permanent ; 
selection  of  longer  and  shorter  specimens  within  it  was 
without  further  effect.  Two  other  long-bodied  races  were 
obtained  in  later  extensive  selections.  Similarly,  from  among 
a  great  number  of  selections  of  peculiarly  shaped  yeast  cells, 
a  number  of  new  races  were  obtained  in  which  the  cells 
were  narrow  and  elongated,  as  compared  with  the  more 
nearly  spherical  cells  of  the  parents. 

What  caused  the  production  of  these  new  races  is  not 
known,  but  they  demonstrate  that  in  bacteria  and  yeasts  at 
times  'the  inherited  characteristics  of  a  race  become  altered. 

More  definite  results  have  been  reached  in  the  study  of 
color  changes  in  bacteria.  The  organism  known  as  BaciUus 
prodigiosus  produces  a  bright  red  color;  it  is  supposed  to 
be  the  cause  of  the  "miracles"  in  which  the  bread  of  the  host 
appears  to  become  bloody.  Wolf  (1909)  attempted  by 
various  means  to  obtain  from  this  organism  races  that  give 
a  different  color  or  that  are  colorless.  A  series  of  fifty 
successive  selections  of  the  lightest  parts  of  the  colonies 
produced  no  inherited  effects ;  the  descendants  were  still  of 
the  typical  color. 

Wolf  further  tried  cultivating  the  colored  bacilli  on  media 
containing  chemicals  of  various  sorts.  He  employed  in  dif- 
ferent cases  copper  sulphate,  potassium  bichromate,  carbolic 


Production  of  Heritable    Variations   in  Bacteria     89 

acid,  corrosive  sublimate,  and  other  metallic  salts.  It  was 
not  at  all  difficult  to  cause  the  organisms  to  lose  color  when 
cultivated  with  these  chemicals ;  keeping  them  at  a  high 
temperature  had  the  same  effect.  But  in  most  cases,  as  soon 
as  the  organisms  were  returned  to  natural  conditions  the 
normal  production  of  color  was  resumed;  the  "acquired 
character"  was  not  inherited.  Similarly  transitory  modi- 
fications of  the  color  in  other  directions  were  produced. 
'Such  results  were  reached  with  infinite  pains  in  a  great  num- 
ber of  experiments,  with  this  organism  and  with  other 
colored  bacteria. 

But  in  cultures  in  which  the  nutritive  medium  contained 
potassium  bichromate,  certain  white  colonies  appeared  which, 
when  transferred  to  media  without  the  chemical,  continued 
to  remain  white,  though  there  appeared  also  red  spots  amid 
the  white.  A  long  series  of  selections  was  carried  on,  choos- 
ing always  the  whitest  parts  of  the  colonies,  but  the  tend- 
ency to  return  partly  to  the  red  condition  could  not  be 
gotten  rid  of  by  selection.  It  was  found  that  the  longer  the 
organisms  were  cultivated  with  potassium  bichromate,  the 
more  firmly  was  the  white  established.  When  it  first  ap- 
peared the  white  color  disappeared  again  as  soon  as  the 
organisms  were  restored  to  normal  surroundings ;  later  the 
white  became  hereditary,  though  there  was  always  a  tendency 
for  some  part  of  the  colonies  to  produce  the  red  color. 
Some  similar  results  were  reached  also  with  other  chem- 
icals. 

In  these  cases  therefore  we  have  a  most  interesting  tran- 
sitional condition.  The  hereditary  character  of  the  race 
has  been  changed,  for  now  the  colonies  are  largely  white 
under  the  same  conditions  in  which  they  were  formerly 
red.  But  they  still  show  a  tendency  to  return  to  the  original 
character. 

But  with  another  substance,  corrosive  sublimate,  a  white 


90  Life  and  Death,  Heredity  and  Evolution 

color  was  produced  that  was  permanent.  When  the  bac- 
teria were  restored  to  their  natural  conditions  they  re- 
mained white,  no  matter  how  long  the  culture  was  continued. 
And  with  certain  other  chemicals  the  bacterial  color  became 
permanently  a  darker  red;  although  restored  to  normal 
conditions  and  kept  there  for  hundreds  of  generations,  the 
acquired  dark  color  persisted. 

This  work  proves  therefore  that  in  bacteria  by  the  action 
of  the  environment  definite  changes  that  are  hereditary  can 
be  produced.  From  a  single  race,  by  subjecting  parts  of  it 
to  these  diverse  agents,  a  number  of  hereditarily  diverse 
races  are  obtained. 

In  this  case  the  alteration  is  evidently  a  change  in  the 
chemical  processes  of  the  organisms.  The  red  color  of  this 
bacterium  is  not  in  the  body  of  the  creature,  but  is  due  to 
some  substance  produced  by  it,  which  colors  the  material  on 
which  the  organisms  live.  In  the  experiments  the  effective 
substances  changed  the  chemical  processes  so  that  the 
bacteria  no  longer  produced  this  substance,  or  produced 
one  of  another  color. 

Similar  in  the  fact  that  they  deal  with  peculiarities  that 
are  visible  to  the  eye  are  certain  experiments  of  Toenniessen 
(1915).  He  investigated  a  certain  strain  of  the  bacillus 
which  produces  pneumonia.  This  organism  produces  a  quan- 
tity of  mucus,  which  forms  a  thick  envelope  in  which  the 
cell  is  imbedded;  the  volume  of  this  envelope  is  perhaps 
several  hundred  times  that  of  the  cell  itself.  When  the 
organisms  are  cultivated  for  a  long  time  in  dense  colonies  on 
agar,  the  products  of  their  nutritive  processes  collect,  until 
they  decrease  the  organisms'  power  to  produce  the  envelope 
of  mucus.  After  a  time  some  of  the  bacteria  are  found  with 
only  a  thin  envelope,  others  with  none  at  all. 

If  these  modified  bacteria  are  transferred  to  normal  con- 


Production  of  Heritable  Variations  in  Bacteria       91 

ditions,  in  which  the  products  of  metabolism  are  not  allowed 
to  gather,  they  usually  at  once  produce  the  normal  amount 
of  mucus ;  the  change  was  not  a  hereditary  one.  But  if  the 
organisms  are  kept  for  a  long  time  under  the  unfavorable 
conditions  (four  weeks  or  more),  some  produce  no  mucus 
at  all;  and  if  these  are  restored  to  normal  conditions,  they 
and  their  descendants  continue  to  be  without  the  mucous 
envelope.  The  change  has  become  hereditary.  But  it  is  still 
not  permanent,  for  by  special  means  the  organisms  can  be 
caused  to  begin  anew  to  produce  the  normal  amount  of 
mucus.  This  is  most  completely  brought  about  through 
allowing  the  organisms  to  live  for  a  time  in  a  living  animal, 
by  infecting  a  white  mouse.  After  passage  through  the 
animal's  body  the  bacteria  have  regained  their  normal 
powers  of  producing  the  mucous  envelope. 

By  long  continued  cultivation  with  the  products  of  metab- 
olism, using  special  methods,  Toenniessen  produced  other 
changes  that  were  permanently  hereditary.  The  organisms 
gradually  produced  less  and  less  mucus,  so  that  successive 
gradations  could  be  distinguished.  At  least  three  of  the 
grades  were  independently  hereditary;  one  had  a  mucous 
envelope  a  little  smaller  than  normal;  a  second  had  a  very 
small  envelope;  the  third  had  no  envelope  whatever.  Long 
continued  cultivation  under  normal  conditions  left  each  of 
these  three  grades  unchanged;  even  passage  through  the 
body  of  animals  did  not  restore  the  organisms  to  the  normal 
condition.  The  alterations  produced  were  permanently  in- 
herited. 

In  this  case,  as  in  the  former,  we  observe  the  striking  fact 
that  what  seems  outwardly  the  same  modification  may  appear 
sometimes  without  being  hereditary;  sometimes  as  heredi- 
tary for  a  number  of  generations ;  sometimes  as  permanently 
inherited.  The  difference  appears  to  depend  on  the  length 


92  Life  and  Death,  Heredity  and  Evolution 

of  time  that  the  modifying  factors  have  acted;  the  longer 
they  act,  the  more  decidedly  hereditary  become  the  changes 
they  produce. 

Many  of  the  hereditary  changes  that  have  been  produced 
in  bacteria  manifest  themselves  only  in  altered  physiological 
activities.  Bacteria  break  up  many  sorts  of  organic  com- 
pounds, obtaining  by  the  recombination  of  their  components 
the  energy  necessary  for  their  own  vital  activities.  Diverse 
species  or  races  thus  decompose  different  compounds.  In  a 
number  of  cases  it  has  been  found  that  if  bacteria  of  a 
particular  sort  are  cultivated  in  the  presence  of  a  com- 
pound which  they  do  not  decompose,  but  which  is  not  too 
unlike  some  compound  on  which  they  can  live,  in  the  course 
of  time  some  of  the  individuals  acquire  the  power  of  de- 
composing and  living  upon  this  unaccustomed  substance. 
This  power  then  remains  hereditary,  so  that  the  descend- 
ants have  it  also, — even  though  they  may  be  cultivated 
under  conditions  in  which  it  is  not  exercised. 

For  example,  Massini  (1907)  found  that  a  certain  bac- 
terium which  belongs  to  the  group  of  which  the  typhoid 
bacillus  is  a  member,  had  not  the  power  of  decomposing 
lactose.  But  if  they  are  grown  on  a  culture  medium  that 
contains  lactose,  after  a  few  days  certain  parts  of  the 
colonies  begin  to  grow  rapidly,  forming  small  nodules ;  and 
tests  show  that  these  are  now  decomposing  the  lactose.  If 
these  are  removed  to  other  media  and  cultivated  for  many 
generations  without  lactose,  their  descendants  still  retain 
the  power  of  decomposing  this  substance,  as  is  shown  by 
replacing  them  on  a  medium  with  lactose. 

This  fact  has  been  confirmed  by  many  observers,  and  sim- 
ilar changes  have  been  observed  in  other  cases.  Bacteria 
have  been  caused  to  acquire  the  power  of  splitting  up  lactose, 
dulcite,  rhamnose  and  various  other  carbohydrates,  though 


Production  of  Heritable  Variations  in  Bacteria      93 

when  first  cultivated  on  these  substances  they  had  not  this 
power.  In  many  cases  the  cultures  so  tested  have  been  de- 
rived originally  from  a  single  individual,  so  that  there  is  no 
question  but  that  there  has  been  an  actual  change  in  the 
hereditary  capabilities  of  a  single  race. 

In  most  cases  the  change  thus  brought  about  is  per- 
manent; the  descendants  never  lose  the  capability  that  they 
have  acquired.  But  in  some  cases  it  has  been  found  that 
long  cultivation  under  other  conditions  causes  the  descend- 
ants to  lose  the  power  which  their  ancestors  had  acquired.2 

The  slightness  and  delicacy  of  the  hereditary  changes  so 
induced,  and  the  fact  that  they  increase  by  gradations,  is 
shown  in  certain  other  experiments  of  Wolf  (1909).  Cer- 
tain peculiar  organisms  known  as  Myxobacteria  form  dense 
swarms  on  decaying  substances.  A  species  known  as  Myxo- 
coccus  rubescens  thus  forms  circular  red  patches  on  culture 
media.  If  from  a  single  patch  of  these,  two  distinct  cultures 
are  made,  and  the  two  swarms  are  later  allowed  to  come  in 
contact,  they  flow  together  and  form  a  single  swarm.  But 
if  the  swarms  come  from  diverse  but  related  species  they  will 
not  unite,  but  remain  sharply  separate.  Even  within  the 
single  species  named  above  it  was  found  that  swarms  from 
diverse  sources  refuse  to  unite,  so  that  there  are  diversities 
of  race  showing  in  this  behavior.  A  large  number  of  races, 
diverse  according  to  this  test,  were  isolated  from  the  single 
species. 

The  possibility  naturally  suggests  itself  that  such  differ- 
ences can  be  produced  within  a  single  race.  This  was  at- 
tempted by  Quehl  (1906),  and  later  by  Wolf  (1909).  A 
single  swarm  was  divided  into  a  number  of  parts,  which 
were  kept  under  different  conditions,  on  diverse  culture 

*  An  excellent  summary  and  review  of  all  such  cases  up  to  1912,  with 
a  helpful  account  of  methods,  and  important  details,  is  given  in  the 
paper  of  Dobell  (1912). 


94  Life  and  Death,  Heredity  and  Evolution 

media.  Later  the  parts  were  brought  together  again,  under 
uniform  conditions,  to  see  whether  they  would  still  unite, 
or  whether  they  had  become  sufficiently  diverse  to  remain 
distinct. 

It  was  found  that  the  parts  might  be  cultivated  for  a  long 
time  under  diverse  conditions,  without  becoming  so  changed 
as  to  refuse  to  unite.  The  first  investigator  who  studied 
the  matter  did  not  succeed  in  getting  a  single  race  to 
divide  into  two  that  were  diverse. 

Wolf  continued  the  experiments,  keeping  the  parts  sep- 
arated a  longer  time,  and  using  many  diverse  cultural  con- 
ditions ;  in  particular  he  added  various  sorts  of  chemicals  to 
the  different  cultures.  In  this  way  after  long  periods  dif- 
ferences were  produced  within  a  single  race.  An  example 
will  make  clear  the  important  facts.  In  one  experiment  a 
single  original  race  was  divided  into  eight  parts,  which  were 
cultivated  on  diverse  media.  At  intervals  it  is  necessary 
to  transfer  each  stock  to  a  new  lot  of  its  medium,  in  order 
to  keep  the  organisms  healthy,  and  it  is  convenient  to  use 
the  number  of  transfers  made  as  a  measure  of  the  relative 
time  required  for  changes  to  occur.  After  a  few  transfers, 
each  of  the  eight  divisions  was  tried  with  all  the  others,  and 
it  was  found  that  all  would  unite  readily.  This  still  oc- 
curred after  fifteen  transfers.  After  twenty-five  transfers, 
it  was  found  that  a  few  of  the  parts  refused  to  unite  with 
some  of  the  others.  After  thirty  transfers  there  were  re- 
fusals in  more  than  half  of  the  combinations,  and  after 
thirty-five  transfers  each  division  refused  to  unite  with  any 
of  the  others.  Each  of  the  eight  parts  of  the  original  race 
had  now  become  diverse  from  each  of  the  others ;  eight  dif- 
ferent stocks  had  been  produced  from  one.  Before  the 
change  was  complete  there  were  many  transitional  condi- 
tions, in  which  there  was  a  reluctance  to  unite,  without  a 


Production  of  Heritable  Variations  in  Bacteria       95 

complete  refusal;  conditions  in  which  union  was  incomplete, 
and  the  like. 

After  the  differences  had  been  brought  about,  the  colonies 
were  all  restored  to  the  same  culture  medium  and  to  the 
same  other  conditions,  and  cultivated  thus  for  a  long  time. 
They  still  refused  to  unite  when  brought  in  contact.  The 
change  produced  was  hereditary  and  permanent.  After 
fifteen  transfers  under  uniform  conditions, — representing 
hundreds  of  generations  of  the  organisms, — the  diversities 
still  existed. 

Even  when  the  organisms  were  cultivated  separately  for  a 
very  long  time,  but  without  diverse  chemicals  in  the  culture 
media,  they  ultimately  became  diverse.  In  an  experiment 
of  this  sort,  it  required  fifty-six  transfers,  occupying  a  year 
and  a  half,  to  bring  about  hereditary  diversities  within  a 
single  stock. 

When  the  diverse  stocks  thus  produced  were  examined 
under  the  microscope,  no  differences  could  be  detected.  The 
change  was  evidently  in  the  intimate  chemical  processes  of 
the  organisms,  not  showing  in  any  visible  way.  The  case  is 
of  particular  interest  because  it  shows  that  hereditary 
changes  may  arise  in  most  delicate  shadings  which  gradually 
become  more  and  more  marked. 

Besides  the  work  which  we  have  just  described,  there  has 
been  much  experimentation  upon  induced  changes  in  heredi- 
tary characters  of  bacteria,  with  special  relation  to  virul- 
ence, to  immunity,  and  the  like.  The  production  of  "attenu- 
ated" strains  of  bacteria,  weaker  in  their  injurious  effects  on 
other  organisms,  is  a  not  uncommon  practice.  But  most  of 
this  work  has  been  done  without  the  precautions  necessary, 
for  establishing  the  results,  from  a  genetic  standpoint,  as  ac- 
tual cases  of  the  inheritance  of  induced  modifications.  But 
the  recent  critical  work  of  Wolf  and  others,  described  above, 


96  Life  and  Death,  Heredity  and  Evolution 

largely  validates  this  large  mass  of  material ;  it  shows  that 
hereditary  changes  of  the  kind  which  appear  to  occur  in 
much  bacteriological  work,  actually  do  take  place  when 
the  matter  is  studied  with  all  the  required  precautions. 
Summaries  of  much  of  this  work,  with  references  to  the  orig- 
inal papers,  will  be  found  in  the  publications  of  Dobell 
(1912),  Jollos  (1914)  and  Pringsheim  (1910). 

#.     Modifications  of  Inherited  Characters  in  Higher 
Protozoa 

A  relatively  small  amount  of  work  has  of  late  been  done 
on  the  modification  of  inherited  characters  in  the  larger  and 
more  complex  forms  of  Protozoa ;  some  of  the  results  here 
perhaps  throw  light  on  the  nature  of  the  processes  occur- 
ring. 

In  the  parasitic  flagellates  Trypanosoma,  facts  similar  to 
some  of  those  above  described  for  bacteria  have  been  dem- 
onstrated. A  good  review  of  the  facts  has  been  given  by 
Dobell  (1912).  One  case  introduces  a  new  element,  which 
possibly  throws  light  on  certain  general  relations.  The 
trypanosome  possesses,  besides  a  typical  nucleus,  a  small 
body  known  as  the  kinetonucleus  (see  Figure  27,  2).  This 
structure  is  placed  close  to  the  inner  end  of  the  motile 
flagellum,  and  may  have  some  relation  to  the  activity  of 
the  latter.  In  reproduction,  the  kinetonuclei  of  the  two 
progeny  are  formed  by  division  of  the  kinetonucleus  of  the 
parent.  In  Trypanosoma  brucei  cultivated  in  mice,  it  was 
found  that  when  certain  chemicals  were  injected  into  the 
mice,  the  kinetonucleus  of  the  trypanosomes  disappears 
(Figure  27,  1).  The  animals  now  multiply  as  usual,  but 
remain  without  kinetonuclei;  this  continues  indefinitely. 
Thus  by  the  action  of  the  chemicals  a  stock  has  been  ob- 
tained which  differs  structurally  from  the  original  race; 


Production  of  Heritable  Variations  m  Protozoa    97 

and  this  diversity  is  inherited  in  ordinary  reproduction  by 
fission.  The  same  result  has  been  produced  wth  several 
other  species  of  Trypanosoma. 

In  Paramecium,  as  well  as  in  some  other  infusoria,  many 
attempts  have  been  made  to  so  modify  the  organisms  that 
they  will  live  under  conditions  which  normally  kill  them. 


Figure  27.  Trypanosoma  brucei.  1.  Individual  from  which  the 
kinetonucleus  has  been  removed,  by  treatment  with  chemicals.  2. 
Normal  form  (the  kinetonucleus  is  the  dark  body  near  the  lower  end). 
After  Werbitzki,  from  Dobell,  1912. 

All  such  attempts,  if  they  are  successful,  involve  a  change 
in  the  animals  and  the  inheritance  of  this  change  by  the 
progeny.  For  since  they  reproduce  every  twenty-four  hours 
or  oftener,  the  acclimatization  would  not  last  longer  than 
that  period,  if  the  progeny  returned  at  once  to  the  original 
condition. 

Most  experiments  in  acclimatization  attempt  to  accustom 
the  organisms  to  high  temperatures ;  or  to  poisons  of  various 
kinds.  One  sometimes  gets,  from  reading,  the  impression 
that  it  is  easy  to  do  this.  But  most  persons  who  try  it  are 
greatly  disappointed.  The  organisms  appear  quite  un- 
changing; if  the  experiments  are  not  carried  on  for  a  very 
long  time,  and  the  change  of  conditions  made  with  extreme 


98  Life  and  Death,  Heredity  and  Evolution 

slowness  and  gradualness,  the  animals  usually  show  no  ac- 
climatization ;  they  die  as  soon  as  the  temperature  or  the 
poison  reaches  the  intensity  which  was  destructive  to  them 
at  the  beginning.  But  with  extreme  patience  and  perse- 
verance, a  change  gradually  appears.  Perhaps  the  most 
thorough  experiment  of  this  sort  ever  made  was  carried  out 
long  ago  by  Dallinger  (1887)  ;  he  continued  the  process  of 
acclimatizing  the  animals  to  higher  temperature  for  seven 
years,  and  reached  more  striking  results  than  anyone  else 
has  attained. 


Figure  28.  Organisms  used  in  Dallinger's  experiment  on  the  effects 
of  high  temperatures.  1,  Monas  Dallingeri;  2,  Dallingeria  Drysdali; 
3,  Tetramitus  rostratus.  After  Dallinger. 

Dallinger  worked  with  three  minute  flagellates  that  live 
in  putrefying  infusions :  Tetramitus  rostratus,  Monas  Dal- 
Imgeri,  and  Dallingeria  Drysdali  (see  Figure  28).  The 
temperature  at  which  they  flourished  was  60°  F.  (16°  C.); 
they  were  killed  at  once  by  a  temperature  of  142°  F.  (61° 
C.).  But  at  least  one  of  them,  Dallingeria,  formed  spores 
which  could  resist  (in  fluids)  a  temperature  of  220°  F. 
(104°  C.). 

Dallinger  undertook  to  accustom  the  animals  to  higher 
temperatures.  He  found  that  up  to  70°  F.  little  difference 
was  observable  in  the  life  and  growth,  although  the  animals 
lived  better  under  the  later  increases  if  the  change  from  60° 
to  70°  was  made  very  slowly.  Above  70°  it  became  neces- 
sary to  proceed  with  extreme  slowness ;  Dallinger  raised  the 


Production  of  Heritable  Variations  in  Protozoa     99 

temperature  by  only  two  degrees  each  month.  At  73° 
many  died,  but  as  the  temperature  remained  at  this  point 
for  two  months,  the  remainder  recovered  their  vigor.  At 
78°  a  critical  point  was  reached;  as  many  of  the  animals 
died,  the  temperature  was  lowered  to  77°  till  there  was 
recovery,  and  by  repeatedly  alternating  the  temperature 
between  these  two  points,  in  eight  months  the  animals  lived 
healthily  at  78°.  They  now  underwent  a  most  interesting 
visible  change ;  the  protoplasm  became  filled  with  small 
vacuoles.  These  continued  for  a  month  or  two,  then  the 
vacuoles  disappeared.  Now  the  temperature  could  be  far- 
ther increased;  in  three  months  it  was  raised  to  80°  F. 

By  a  continuation  of  this  slow  and  painful  process  the 
animals  were  finally  brought  to  live  vigorously  at  a  tem- 
perature of  158°  F.  (70°  C.).  There  were  repeated  critical 
points,  at  which  the  animals  had  to  be  kept  for  months 
before  further  advance  could  be  made.  In  several  of  these 
there  was  a  renewed  formation  of  vacuoles  in  the  protoplasm, 
the  vacuoles  finally  disappearing.  After  these  periods  the 
raising  of  the  temperature  could  continue  more  rapidly. 
To  bring  the  animals  to  158  degrees,  seven  years  were  re- 
quired. The  experiment  was  then  most  unfortunately  ended 
by  an  accident. 

No  such  long  continued  experiment  has  ever  been  carried 
through  since  this  work  of  Dallinger.  It  is  clear  that  not 
only  had  the  organisms  of  a  given  generation  been  changed, 
but  they  transmitted  the  change  to  their  offspring.  For  at 
the  end  of  any  period  of  24  hours  a  totally  new  generation 
was  present.  At  the  beginning  of  the  experiment  all  the 
individuals  were  destroyed  by  a  temperature  of  78° ;  while 
at  the  end  they  lived  and  flourished  at  a  temperature  above 
150°. 

Yet  it  is  to  be  remembered  that  even  at  the  beginning 


100         Life  and  Death,  Heredity  and  Evolution 

the  animals  could  form  spores  which  resisted  a  much  higher 
temperature  (242°)  than  that  to  which  the  active  animals 
were  finally  accustomed.  In- transforming  from  active  ani- 
mals to  spores,  the  protoplasm  must  go  through  some 
process  which  makes  it  more  resistant  to  heat.  It  seems 
probable  that  during  the  acclimatization  the  protoplasm 
of  the  active  animals  went  through  a  similar  process.  It 
has  been  suggested  that  the  essential  point  in  both  cases  is 
the  getting  rid  of  a  certain  proportion  of  the  water  in  the 
protoplasm,  leaving  it  denser,  for  protoplasm  containing  lit- 
tle water  is  as  a  rule  less  injured  by  heat  than  when  it 
contains  much  water.  This  new  physical  condition  of  the 
protoplasm  must  then  have  persisted  through  reproduction, 
and  so  been  handed  on  to  the  offspring. 

Some  effects  of  temperature  in  altering  a  different  mani- 
festation of  the  hereditary  constitution  have  recently  been 
studied  by  Middleton  (1918).  Progeny  of  a  given  indi- 
vidual of  the  infusorian  Stylonychia  pustulata  were  divided 
into  two  sets ;  one  set  was  kept  at  a  high  temperature,  the 
other  at  a  low  temperature.  Those  at  the  high  tempera- 
ture (about  30°  C.)  divided  more  rapidly  than  those  at 
the  low  temperature  (about  10°  C.).  After  various  inter- 
vals, members  of  the  two  sets  were  brought  to  a  common 
intermediate  temperature,  and  their  rates  of  fission  com- 
pared. 

It  was  found  that  the  stay  in  diverse  temperatures  had 
altered  the  hereditary  constitution  so  as  to  give  diverse 
rates  of  fission  in  the  two  stocks.  After  about  thirty  days 
in  the  different  temperatures,  the  set  that  had  been  kept 
at  high  temperatures  continued  to  divide  more  rapidly  than 
the  others,  even  though  both  were  now  at  the  same  tempera- 
ture. But  after  longer  periods  in  the  diverse  temperatures, 
— after  two  or  three  months  or  more, — there  was  a  change 


Production  of  Heritable  Variations  in  Protozoa     101 

in  the  inherited  effects.  Now  when  both  sets  were  placed  at 
intermediate  temperatures,  the  set  that  had  been  kept  at  the 
higher  temperature  divided  less  rapidly  than  the  set  that 
had  been  kept  at  low  temperature.  This  difference  per- 
sisted for  as  long  a  period  as  the  stocks  were  retained, — - 
about  two  months. 

Other  evidence  showed  that  the  high  temperature  grad- 
ually injured  the  stock,  so  that  in  the  course  of  time  the 
high  temperature  set  came  to  divide  less  rapidly  even  while 
subjected  to  high  temperature.  At  the  end  of  six  months 
those  kept  at  the  high  temperature  all  died  out,  while  the 
other  set  was  still  vigorous.  The  persistent  low  fission  rate 
of  the  high  temperature  set  when  restored  to  normal  tem- 
perature was  apparently  a  manifestation  of  this  injury. 
The  latter,  whatever  its  nature,  was  inherited  in  the  vege- 
tative reproduction. 

Dallinger  did  not  determine  how  long  the  acquired  re- 
sistance to  heat  would  have  lasted  if  his  animals  had  been 
restored  gradually  to  lower  temperatures ;  and  no  study  for 
long  periods  of  the  permanence  of  the  effects  observed  by 
him  was  made  by  Middleton.  But  this  matter  has  been 
studied  particularly  in  Paramecium,  by  Jollos  (1913  a, 
1914).  He  attempted  to  accustom  Paramecium  caudatum 
to  higher  temperatures,  and  to  increased  concentration  of 
certain  compounds  of  arsenic.  Some  races  resisted  acclima- 
tization completely.  In  others  after  long  periods  the  ani- 
mals could  stand  somewhat  higher  temperatures  or  higher 
concentrations  than  before.  But  when  they  were  returned  to 
the  normal  conditions,  they  lost  their  acquired  immunity  al- 
most at  once. 

In  other  cases  the  animals  acquired  a  resistance  to  poisons 
which  was  retained  by  their  descendants  for  many  genera- 
tions. Thus,  in  a  certain  race  B  the  animals  were  killed 


102          Life  and  Death,  Heredity  and  Evolution 

when  1.1  parts  of  a  standard  solution  of  arsenic  was  added 
to  100  parts  of  the  water.  By  a  gradual  process  they  were 
rendered  resistant  to  5  parts  of  this  same  solution  to  100 
of  water.  When  they  were  again  cultivated  in  fluid  without 
arsenic,  they  retained  their  resistance  unchanged  for  seven 
months,  or  at  least  200  generations.  But  in  the  eighth 
month  it  was  found  that  the  resistance  was  partly  lost; 
they  could  now  stand  only  4  parts  of  the  arsenical  solution 
in  100.  The  resistance  continued  to  decrease  gradually, 
until  at  the  end  of  ten  and  a  half  months  they  had  entirely 
lost  the  resistance  to  arsenic  that  they  had  acquired,  so 
that  they  were  killed  by  the  same  weak  doses  that  had 
been  destructive  at  the  beginning.  In  many  other  cases 
Jollos  thus  produced  modifications  of  the  power  of  resisting 
chemicals,  which  thus  lasted  for  months,  but  finally  disap- 
peared. He  found  that  if  the  animals  were  kept  under  con- 
stant conditions  their  resistance  lasted  much  longer  than 
was  the  case  if  they  were  subjected  to  many  changes  of 
temperature  and  food. 

One  particular  phenomenon  did  away  immediately  with 
the  acquired  resistance ;  this  was  conjugation.  Jollos  found 
that  after  the  animals  had  acquired  resistance  to  a  consid- 
erable concentration  of  arsenic,  this  resistance  was  com- 
pletely lost  as  soon  as  they  conjugated.  To  this  fact  Jollos 
attributes  a  deep  significance.  He  believes  that  it  shows  that 
the  modifications  thus  produced  and  for  a  long  time  passed 
from  parent  to  offspring  by  fission  are  in  reality  very  dif- 
ferent things  from  the  permanently  inherited  characteristics 
of  the  species.  These  characters — the  typical  form,  struc- 
ture, and  physiology — are  inherited  not  only  in  fission,  but 
also  in  the  changes  which  follow  upon  conjugation.  A 
change  in  these  characters, — a  permanent  change  in  the 
inheritance, — Jollos  would  call  a  mutation,  while  these 


Nature  of  Heritable  Variations  103 

changes  that  are  handed  on  only  through  vegetative  repro- 
duction he  calls  modifications ;  the  two  he  believes  to  be  of 
essentially  diverse  nature.  Such  a  "mutation"  Jollos  be- 
lieves that  he  saw  in  a  single  instance  in  Paramecium.  In 
one  of  his  cultures  kept  at  high  temperatures  there  ap- 
peared individuals  which  were  much  more  resistant  to  heat 
than  most  of  the  animals;  they  could  be  cultivated  at  39° 
C.,  which  soon  killed  the  others.  These  individuals  retained 
their  high  resistance  even  after  conjugation;  it  had  become 
a  permanently  inherited  character.  In  no  other  case  was  a 
modification  retained  through  conjugation.  Jollos  holds 
that  practically  all  the  changes  in  bacteria  and  other  Pro- 
tista, which  we  have  described  above,  are  merely  instances 
of  these  temporary  modifications. 

That  there  is  such  a  difference  in  principle  between  the 
two  things, — between  modifications  that  are  passed  on  only 
in  fission,  but  disappear  as  soon  as  there  is  sexual  reproduc- 
tion, so  that  they  cannot  be  said  to  form  part  of  the  really 
hereditary  characters  of  the  stock, — between  these  and  the 
really  hereditary  characters, — cannot  yet  be  considered  es- 
tablished. If  there  is  such  a  difference,  one  can  hardly  re- 
frain from  bringing  it  in  some  way  into  relation  with  the 
two  nuclei.  Since  in  fission  the  new  active  nuclei  are  pro- 
duced by  division  of  the  active  nucleus  of  the  parent,  one 
might  naturally  assume  that  the  seat  of  the  temporary 
modifications  is  in  the  active  or  macronucleus,  while  the  re- 
serve nucleus  (micronucleus)  has  not  been  affected.  Thus 
would  be  accounted  for  the  fact  that  at  conjugation,  when 
the  macronucleus  disappears  and  is  replaced  by  the  mi- 
cronucleus, the  modifications  also  disappear;  they  go  with 
the  macronucleus.  But  we  now  know  from  the  work  of 
Woodruff  and  Erdmann  that  the  macronucleus  disappears 
and  is  replaced  from  the  micronucleus  at  intervals  even 


104          Life  and  Death,  Heredity  and  Evolution 

without  conjugation;  the  modifications  should  therefore  dis- 
appear at  such  times.  The  fact  that  they  do  not  indicates 
that  the  distinction  is  not  one  depending  on  whether  the  seat 
of  the  modifications  is  in  the  macronucleus  or  the  micronu- 
cleus ;  it  leaves  the  distinction  indeed  with  no  very  intelligible 
foundation.  It  appears  possible  that  modifications  which 
are  novv  known  to  last  for  months  might  endure  still  longer, 
and  become  as  permanent  as  any  character,  if  the  conditions 
producing  them  lasted  for  much  longer  periods.  The  fact 
that  the  modifications  sometimes  disappear  at  conjugation 
may  be  due  to  the  fact  that  variations  of  many  sorts  occur 
as  a  result  of  conjugation,  as  will  be  set  forth  in  our  account 
of  that  matter.  The  number  of  cases  in  which  these  phe- 
nomena have  been  studied  is  very  small ;  too  small  for  basing 
positive  conclusions  on  these  points. 

All  together,  the  studies  of  the  effects  of  external  agents 
on  heredity  in  the  Protozoa  show  that  changes  in  the 
hereditary  characters  are  in  this  way  produced  only  most 
slowly  and  rarely.  The  organisms  are  found  most  resistant 
to  such  changes ;  any  alteration  produced  in  a  given  genera- 
tion is  usually  compensated  for  in  the  next  generation.  Al- 
most every  investigator  of  the  matter  passes  through  a  long 
stage  in  which  he  can  hardly  resist  the  conviction  that  no 
hereditary  changes  can  be  brought  about  in  this  manner. 
But  if  work  is  continued  for  very  long  periods  of  time,  the 
hereditary  constitution  of  the  stock  is  seen  to  gradually 
yield;  at  first  only  in  a  slight  degree  and  with  results  that 
are  transitory.  The  differences  finally  become  so  fixed  that 
they  are  transmitted  in  the  ordinary  reproduction  by  fission. 
After  conjugation,  with  its  extreme  physiological  altera- 
tions, and  production  of  new  combinations  of  inherited  char- 
acters, the  inherited  environmental  effects  are  frequently  no 


Nature  of  Heritable  Variations  105 

longer  clearly  in  evidence;  possibly  they  are  masked  by  the 
new  combinations  occurring;  possibly  actually  lost. 

In  general  the  results  of  the  work  suggest  that  the  many 
slightly  differing  stocks  found  in  any  one  of  these  lower  or- 
ganisms may  owe  their  origin  partly  to  the  inherited  effects 
of  long  continued  environmental  diversities. 


The  Natural  History  of  Mating.  Sex,  Its  Nature  and 
Consequences.  Sex  in  the  Protozoa:  Is  Sex  Coextensive 
•with  Life  and  Necessary  to  Its  Continuance? 

have  dealt  with  heredity  and  other  genetic  problems 
in  the  cases  where  there  is  but  a  single  parent;  we 
now  turn  to  reproduction  where  there  are  two  parents  in- 
stead of  one.  The  mating  of  two  individuals  that  occurs  at 
times  in  almost  all  organisms  is  one  of  the  most  extraor- 
dinary processes  in  nature;  it  has  the  effect  of  complicat- 
ing tremendously  all  biological  questions.  Volumes  have 
been  written  as  to  its  purpose  and  meaning. 

Some  tell  us  that  it  is  unscientific  to  ask  as  to  the  "pur- 
pose" or  "object"  of  any  process;  Dobell  (1914)  has  made 
this  point  with  relation  to  all  such  discussions  of  mating. 
The  criticism  is  justified,  so  far  as  the  method  of  expression 
goes,  and  the  literal  implications  of  that  method  of  expres- 
sion; science  cannot  deal  with  purposes  or  ends,  save  in  the 
case  of  conscious  human  purposes.  Nevertheless,  a  really 
scientific  question  is  often  hidden  under  this  form  of  expres- 
sion. What  it  really  means  is:  What  difference  does  it 
make  whether  this  process  occurs  or  not?  Any  question 
as  to  "purpose"  or  "object"  that  can  be  put  in  this  form  is 
a  scientific  question  in  spite  of  its  teleological  clothing;  any 
teleological  question  that  cannot  be  put  in  this  form  is  no 
affair  of  science.  To  ask  what  difference  this  phenomenon 
makes,  leads  at  once  to  experiment;  a  question  that  could 
106 


Effect  of  Mating  107 

not  be  settled  by  any  conceivable  experiment  is  not  part  of 
science. 

So  the  question  in  which  men  have  been  interested  in  re- 
lation to  mating  and  fertilization  is:  What  difference  does 
it  make  whether  this  occurs  or  not?  This  is  strictly  a  ques- 
tion of  observation  and  experiment,  on  the  same  footing1  as 
the  question :  What  difference  does  it  make  whether  animals 
take  food  or  not? 

When  we  ask  this  question  regarding  the  union  of  two 
individuals  or  parts  of  individuals  which  we  call  mating  and 
fertilization,  we  find  that  there  is  hardly  another  phenomenon 
in  biology  that  so  alters  the  whole  face  of  things.  Biology 
would  be  a  relatively  simple  subject  if  there  were  no  periodic 
unions  of  diverse  individuals,  with  the  accompanying  proc- 
esses. This  union  has  results  of  so  many  different  kinds, 
some  immediate  and  obvious,  others  remote  and  hidden,  that 
we  find  little  agreement  in  the  accounts  of  its  fundamental 
features  given  by  different  investigators. 

A  picture  of  what  happens  in  the  higher  organisms  that 
we  are  familiar  with  will  bring  the  question  sharply  before 
us.  Mating  here  involves  two  diverse  individuals  that  we 
call  male  and  female,  and  two  diverse  germ  cells,  which  we 
likewise  call  male  and  female.  But  this  is  not  the  end; 
the  final  mating  is  between  certain  parts  of  the  cell,  after 
the  two  germ  cells  have  joined  to  form  one.  The  cell 
now  contains  a  set  of  pairs  of  small  visible  packets  of 
chemicals,  the  chromosomes  (Figure  29).  These  mate  in 
pairs  (Figure  29,  D,  E,  F)  and  again  separate.  This  is  the 
final  and  elementary  action  of  mating;  the  union  of  these 
chromosomes  contains  the  secret  of  sex  and  of  mating. 
The  union  of  the  two  diverse  germ  cells  forms  the  starting 
point  for  the  development  of  the  new  individual. 

Several  questions  of  general  interest  come  into  view  in 


108  Life  and  Death,  Heredity  and  Evolution 


6Q 


Figure  29.  Chromosomes  and  their  mating.  A.  Nucleus  containing 
the  chromosomes,  from  the  salamander.  B.  The  23  chromosomes  in  a 
single  cell  of  a  male  grasshopper,  as  seen  under  the  microscope.  C. 
The  chromosomes  of  B  drawn  separately  and  arranged  so  as  to 
show  that  the  group  consists  of  a  series  of  12,  the  two  members  of 
each  pair  being  of  the  same  size  and  form.  One  chromosome  (fifth 
from  the  left  in  the  upper  row)  is  without  a  mate  in  the  male;  in  the 
cell  of  a  female  it  has  a  mate.  D.  The  members  of  the  pairs  after 
mating.  Each  of  the  12  structures  (save  one)  is  formed  by  the  union 
of  the  two  members  of  a  pair.  E  and  F.  Details  of  the  mating  of  the 
chromosomes  in  the  cells  of  another  species  of  grasshopper.  In  E  only 
two  chromosome  pairs  are  seen;  in  the  pair  to  the  right  mating  side 
by  side  has  begun,  but  is  not  complete.  In  F  several  pairs  are  shown, 
fully  mated.  Each  of  the  granules  of  .which  the  chromosome  is  com- 
posed mates  with  a  granule  of  corresponding  size  and  position  in  the 
other  chromosome.  B  to  D,  after  Robertson,  1908.  E  and  F,  after 
Wenrich,  1916. 


Nature  of  Sex  109 

regard  to  this  strange  process.  First,  is  this  union  a  neces- 
sity of  life?  That  is,  could  and  would  life  continue  with- 
out its  occurrence?  Could  and  would  development  occur 
without  it?  Is  there  some  single  general  result  produced 
by  mating, — something  as  general  as  the  production  of 
energy  through  the  taking  of  food?  If  there  is,  what  is  this 
single  general  effect  of  mating?  This  is  the  real  point 
underlying  the  much-discussed  question:  What  is  the  pur- 
pose of  mating? 

Certain  other  questions  arise  from  the  differences  between 
the  two  sexes.  What  is  the  nature  of  this  difference?  That 
is,  is  there  a  single  kind  of  chemical  or  physiological  dif- 
ference between  the  sexes,  wherever  sex  differences  occur;  a 
fundamental  diversity  of  which  all  other  sex  diversities  are 
consequences?  And  does  this  diversity  exist  whenever  there 
is  union  of  individuals  or  cells  or  parts  of  cells?  'Are  the 
two  chromosomes  that  unite  diverse  in  this  manner?  And 
are  all  the  unions  that  occur  a  consequence  of  such  diver- 
sity? Or  may  two  precisely  similar  individuals  or  cells  or 
chromosomes  unite  at  mating?  Again,  is  this  sex  differ- 
ence coextensive  with  life,  so  that  all  living  things  are  com- 
posed of  two  classes  of  substance,  male  and  female,  as 
some  have  asserted?  Or  is  sex  difference  something  that 
pertains  only  to  certain  kinds  of  organisms ;  perhaps  some- 
thing that  has  arisen  during  evolution,  like  the  difference 
between  two  species  of  plants  or  animals? 

We  shall  examine  the  facts  in  the  Protozoa  in  their  bear- 
ing on  these  questions,  and  shall  try  to  determine  with  which 
answers  these  facts  agree  best.  We  will  take  up  the  process 
of  union  first  in  what  is  perhaps  the  best  known  and  most 
instructive  case  in  these  lower  organisms,  in  the  infusorian 
Paramecium.  Then  we  shall  compare  what  happens  in  this 
animal  with  what  occurs  in  others,  keeping  in  mind  through- 
out the  fundamental  questions  that  we  have  set  forth. 


110          Life  and  Death,  Heredity  and  Evolution 

As  you  recall,  Paramecium  multiplies  for  many  genera- 
tions in  single  lines,  the  offspring  having  but  one  parent. 
Then  mating  occurs ;  the  animals  place  themselves  with  their 
oral  sides  together  and  become  partly  united  (Figure  6). 
The  surfaces  of  the  two  adhere,  and  at  a  certain  spot  the 
interior  protoplasm  of  the  two  comes  into  actual  union.  The 
main  things  that  then  occur  are  those  shown  in  Figure  8 
(page  26),  and  in  Figure  30.  The  old  active  nucleus  (mac- 
ronucleus)  breaks  in  pieces  and  is  gradually  absorbed,  like 
so  much  food;  it  disappears  completely.  This  really  re- 
quires a  long  time,  so  that  fragments  of  it  are  still  found 
in  later  stages,  but  as  this  has  no  importance,  these  frag- 
ments are  omitted  from  the  figures  in  order  not  to  confuse 
them.  The  single  small  reserve  nucleus  (micronucleus) 
divides  twice,  into  four  (Figure  30,  A,  B,  C),  and  three  of 
these  are  absorbed  and  disappear  (C)  like  the  macronu- 
cleus.  Then  the  remaining  (fourth)  one  divides  into  two 
parts  (see  Figure  40).  Of  these  two  parts,  one  lies  a  little 
nearer  the  surface  of  union  of  the  two  conjugants,  and  is  a 
little  smaller  than  the  other.  This  one  begins  to  move 
toward  the  opposite  conjugant;  it  is  therefore  commonly 
spoken  of  as  the  "migratory"  half  nucleus.  In  the  other 
conjugant  the  same  thing  happens,  so  that  the  two  migra- 
tory half  nuclei  meet  and  pass  each  other  at  the  boundary 
between  the  two  conjugating  individuals  (Figure  40,  B,  C, 
D).  Each  of  the  two  stationary  halves  remains  in  place 
till  the  migratory  half  nucleus  from  the  opposite  individual 
reaches  it ;  then  the  migratory  and  stationary  half  nuclei 
unite  (Figure  30,  E;  Figure  40,  E,  F). 

Thus  the  general  upshot  of  the  process  is  that  the  two 
mating  animals  exchange  halves  of  their  micronuclei,  and 
at  the  end  each  has  a  micronucleus  composed  of  substance 
partly  from  one  mate,  partly  from  the  other.  This  is  evi- 
dently the  central  point  in  the  conjugation. 


Conjugation  of  Paramecium  Caudatum  111 


Figure  30.  Diagram  showing  the  chief  processes  in  the  conjugation 
of  Paramecium  caudatum.  The  larger  black  bodies  are  the  macro- 
nuclei;  the  smaller  ones  the  micronuclei.  The  clear  circles  are  the 
micronuclei  that  disappear.  The  connecting  lines  show  the  origin  by 
division  of  the  various  structures.  After  the  separation  of  the  two 
members  of  the  pair,  at  G,  only  one  of  them  is  followed  farther;  the 
other  goes  through  the  same  processes  (H  to  L). 


112         Life  and  Death,  Heredity  and  Evolution 

After  the  union  of  the  two  half  nuclei  the  two  mates  sep- 
arate; we  may  now  call  each  an  ex-conjugant.  Now  a  set  of 
peculiar  processes  occurs,  with  the  result  of  restoring, 
through  two  divisions  of  each  ex-conjugant,  individuals  hav- 
ing the  same  structure  as  did  the  mates  before  conjugation. 
Some  of  the  details  of  the  process  may  turn  out  of  great 
significance,  though  at  present  their  meaning  is  not  clear. 

At  separation  each  ex-conjugant  has  a  single  nucleus 
formed  by  the  union  of  the  two  half  nuclei  (Figure  30,  F). 
This  single  nucleus  divides,  producing  two ;  these  divide,  pro- 
ducing four;  these  again  divide,  producing  eight.  These 
eight  are  all  present  in  the  single  ex-conjugant  (Figure 
30,  J).  Now  of  these  eight  three  dissolve  and  disappear, 
leaving  five  in  the  single  individual.  One  of  these,  as  it 
later  turns  out,  is  the  nucleus  from  which  all  the  micronuclei 
of  later  generations  arise  by  division;  the  other  four  later 
form  four  macronuclei  of  four  individuals  of  later  genera- 
tions,— each  increasing  greatly  in  size. 

Now  the  single  ex-conjugant  divides,  producing  two  indi- 
viduals. At  the  same  time  the  single  micronucleus  divides 
into  two  halves,  each  passing  to  one  of  the  offspring.  Each 
of  the  two  offspring  also  receives  two  of  the  four  macronu- 
clei, which  are  now  enlarging  (Figure  30,  K).  Next  each 
of  these  two  offspring  divides  anew  with  repetition  of  the 
division  of  the  micronucleus,  while  of  the  four  progeny  each 
receives  one  of  the  four  macronuclei.  Thus  after  two  divi- 
sions of  the  ex-conjugant,  offspring  are  produced  with  a  sin- 
gle macronucleus  and  a  single  micronucleus  (Figure  30,  L), 
— like  the  individuals  before  conjugation.  Each  of  the  four 
individuals  arising  from  an  ex-conjugant  has  one  of  the 
four  macronuclei  that  were  present  in  the  ex-conjugant. 
But  the  micronucleus  of  each  of  the  four  is  derived  by  divi- 
sion from  a  single  micronucleus  present  in  the  ex-conjugant. 


The  Processes  in  Mating  113 

What  significance  is  to  be  attached  to  the  dissolution  of 
three  of  the  eight  nuclei  originally  present  in  the  ex-con- 
jugant  is  not  clear;  nor  is  it  clear  what  is  meant  by  the 
diverse  method  of  origin  and  distribution  of  the  new  mac- 
ronuclei  as  compared  .with  the  new  micronuclei ;  to  this  point 
we  return  later. 

The  many  details  in  these  processes  that  occur  after  sep- 
aration of  the  ex-con jugant  must  not  be  allowed  to  obscure 
the  essential  features.  These  are  simply  that  the  new 
micronucleus  formed  by  union  of  the  half  nuclei  divides  so 
as  to  produce  new  active  macronuclei  and  new  reserve  mi- 
cronuclei (Figure  30,  G  to  L)  ;  and  the  animals  continue  to 
divide  by  fission,  as  they  did  before  conjugation, — each  of 
the  offspring  getting  a  single  active  nucleus  and  a  single 
reserve  nucleus. 

In  this  process  of  conjugation  are  involved  all  the  prob- 
lems of  sex  and  of  mating.  It  is  of  interest  to  examine  it 
in  connection  with  the  general  questions  which  we  proposed 
in  our  introduction  to  this  lecture,  and  with  some  of  the 
commoner  answers  to  these  questions.  To  this  we  turn. 

What  are  the  results  of  this  mating?  What  difference 
does  it  make  to  the  organisms  or  to  the  race  which  they 
make  up? 

As  we  saw  in  our  first  lecture,  the  best  known  theory  as 
to  the  effect  of  such  conjugation  is  that  it  rejuvenates  the 
organism;  that  it  gets  rid  of  the  effects  of  age.  But  in 
what  way  can  it  have  this  effect?  There  are  two  possible 
answers  to  this  question.  One  is  that  the  rejuvenescence  is 
a  result  of  the  replacement  of  the  old  active  nucleus  by  the 
reserve  nucleus.  But  for  this  no  union  of  two  individuals 
is  necessary,  either  logically  or  in  fact ;  we  know  now  that 
such  replacement  occurs  without  union.  This  answer  there- 
fore gives  no  explanation  of  the  fact  of  union ;  what  we  are 


114          Life  and  Death,  Heredity  and  Evolution 

interested  in  now  is  precisely  the  question  as  to  the  effect 
of  the  mating  as  distinguished  from  the  process  of  replace- 
ment. A  second  answer  to  the  question  as  to  how  rejuvenes- 
cence is  brought  about  holds  that  it  is  due  primarily  to  this 
union  of  two  individuals  or  of  two  nuclei.  In  the  minds  of 
many  that  hold  it  the  grounds  for  this  belief  are  undefined; 
Maupas  (1889,  p.  486)  in  his  great  work  on  rejuvenescence, 
in  which  he  maintains  this  theory,  avows  distinctly  that  he 
cannot  see  how  the  union  of  nuclei  should  produce  re- 
juvenescence, though  he  believes  that  it  does.  But  there  has 
grown  up  a  theory  as  to  how  the  rejuvenescence  is  brought 
about  by  union ;  a  theory  that  is  rather  generally  held.  This 
theory  depends  upon  the  farther  theory  that  there  is  a 
fundamental  difference  between  the  male  and  female  sexes, 
and  that  this  essential  sex  difference  is  always  present  in 
mating,  and  is  the  basis  of  rejuvenescence.  We  know  indeed 
that  in  many  organisms  the  differences  between  the  sexes  are 
not  mere  superficial  diversities,  but  that  the  two  sexes  differ 
in  every  cell  of  their  bodies,  and  the  observable  difference 
lies  precisely  in  the  nuclei,  which  we  know  to  be  funda- 
mentally important  parts  of  the  organism.  In  the  com- 
moner cases,  including  man,  the  nuclei  of  the  female  have  one 
more  chromosome  than  have  those  of  the  male  (Figure  31, 
A  and  B)  ;  in  other  cases  one  chromosome  of  the  female 
differs  from  the  corresponding  one  in  the  male  (Figure 
31,  C  and  D).  Such  a  visible  structural  difference  neces- 
sarily means  further  a  diversity  in  the  most  fundamental 
and  intimate  physiological  processes  of  the  two  sexes;  in 
the  chemical  changes  that  determine  the  nature  of  life. 
These  chemical  changes  we  observe  to  occur  in  the  physio- 
logical interaction  between  the  nucleus  and  the  rest  of  the 
protoplasm. 

What  are  the  fundamental  physiological  differences  be- 


Nature  of  Sexual  Diversity  115 

tween  the  sexes?  In  a  general  way  the  male  in  most  organ- 
isms is  the  more  active  and  the  less  inclined  to  store  up  re- 
serve food  in  its  tissues;  the  female  less  active  and  given 
to  storing  up  more  reserve  nutrition.  These  differences  are 
of  course  seen  most  clearly  in  the  germ  cells  of  the  two 
sexes;  the  typical  male  germ  cells  are  minute  and  actively 
motile,  while  the  female  germ  cells  (eggs)  are  large  and 
inert,  with  much  food  material  stored  in  them.  Less  marked 


Figure  31.  Differences  between  the  chromosomes  of  the  nuclei  in 
the  two  sexes.  A  and  B,  male  and  female  chromosome  groups,  re- 
spectively, of  the  hemipterous  insect  Protenor,  after  Wilson  (1910). 
The  nucleus  of  the  female  (B)  has  two  of  the  large  chromosomes  x, 
while  the  male  (A)  has  but  one. 

C  and  D,  male  and  female  chromosome  groups  respectively,  in  the 
nuclei  of  the  fruit  fly  Drosophila,  after  Morgan  (1916).  The  maid 
group  (C)  has  one  bent  chromosome  (Y)  in  place  of  one  of  the  straight 
ones  (X)  of  the  female  (D). 

differences  of  the  same  general  character  distinguish  the 
male  and  female  individuals  that  produce  these  germ  cells. 

Many  attempts  have  been  made  to  express  such  differences 
in  general  terms.  Geddes  and  Thompson  (1880)  say  that  in 
the  female  the  preponderating  process  is  that  of  anabolism, 


116         Life  and  Death,  Heredity  and  Evolution 

or  the  building  up  of  organic  material,  with  the  storing  up 
of  energy,  while  in  the  male  the  prevailing  process  is  catabo- 
lism,  or  the  breaking  down  process  by  which  energy  is  set 
free, — this  energy  showing  itself  in  greater  movement.  Or 
the  male  has  been  called  "kinetic,"  the  female  "trophic"; 
or  the  male  progressive,  the  female  conservative ;  or  it  is  said 
that  in  the  male  the  animal  functions  prevail,  in  the  female 
the  vegetative  functions. 

In  recent  times  emphasis  has  been  laid  on  a  parallel 
diversity  observable  between  certain  parts  of  the  cell  or 
parts  of  nuclei.  When  the  cell  divides,  certain  parts  seem 
to  actively  initiate  and  carry  on  the  movements  that  bring 
about  division,  while  other  parts  are  passively  moved  by 
these  active  portions.  In  many  organisms  a  structure  called 
the  aster,  which  in  some  cases  itself  appears  to  be  derived 
from  a  small  body  called  the  centrosome,  sets  up  at  the  time 
of  division  a  great  activity  in  the  protoplasm,  forming  the 
spindle,  and  taking  the  lead  in  the  activities  of  cell  division 
(see  Figure  32).  Another  portion  of  the  cell,  comprising 
the  part  commonly  called  the  nucleus,  and  particularly  the 
chromosomes,  is  apparently  passively  moved  by  these  active 
portions.  In  some  Protozoa  there  are  two  nuclei,  one  of 
which,  the  so-called  kinetonucleus  or  blepharoplast  ( see  Fig- 
ure 27),  is  connected  with  the  organs  of  motion,  and  is  sup- 
posed by  some  to  correspond  to  the  centrosome.  The  other, 
the  so-called  trophonucleus,  appears  more  passive,  and  is 
held  to  have  functions  connected  primarily  with  metabolism. 
The  materials  of  which  these  two  parts  are  composed  are 
called  respectively  kinetoplasm  and  trophoplasm,  and  it  is 
held  by  the  upholders  of  this  doctrine  that  these  two  kinds 
of  material  are  present,  either  mingled  or  separate,  in  every 
cell  that  can  divide.1 

1 A  r6sum6  of  the  facts  on  which  this  notion  is  based  is  found  in  the 
paper  of  Dobell  (1909). 


Nature  of  Sexual  Diversity 


117. 


Figure  32.  The  active  and  passive  portions  of  cell  and  nucleus  in 
the  egg  of  the  whitefish.  A.  Egg  just  before  division;  with  centrosome 
(c),  aster  (a),  nucleus  (nu.)  and  cytoplasm  (cy.).  A  set  of  radiations 
have  been  formed  about  the  aster  as  a  center,  indicating  great  activity. 
B.  Later  stage;  the  aster  has  begun  to  divide  into  two.  C.  Still  later; 
the  aster  has  separated  into  two  with  the  nucleus  between  them.  The 
activities  of  the  asters  have  begun  to  invade  the  nucleus.  D.  The 
nucleus  has  been  transformed  by  the  activities  of  the  asters  into  a 
spindle-like  body,  with  the  small  chromosomes  (chr.)  ranged  across  its 
middle. 


These  two  kinds  of  material,  with  their  contrasted  ways 
of  behaving,  are  held  by  many  to  form  the  basis  for  maleness 
and  femaleness,  so  that  sex  diversity  is  coextensive  with  life, 


118          Life  and  Death,  Heredity  and  Evolution 

although  both  sexes  may  be  present  in  a  single  nucleus.  But 
individuals  or  cells  in  which  the  active  material  or  kine- 
toplasm  prevails  are  male,  while  those  in  which  the  nutritive 
material  or  trophoplasm  is  preponderant  are  female. 

According  to  such  theories,  for  vigorous  and  successful 
life,  an  organism  must  have  both  these  substances  or  proper- 
ties,— both  maleness  and  femaleness, — in  fitting  proportions. 
And  it  is  the  gradual  dislocation  of  the  required  proportions 
that  gives  rise  to  the  necessity  of  periodical  unions  of  di- 
verse cells  or  nuclei,  and  to  the  rejuvenescence  that  results 
from  such  unions.  It  is  commonly  held  that  as  life  goes  on, 
certain  individuals  or  cells  or  parts  of  cells  develop,  perhaps 
by  accidental  inequalities  in  division,  farther  and  farther  in 
the  direction  of  maleness,  of  activity,  of  energy-production; 
that  is,  the  kinetoplasm  exceeds  greatly  the  trophoplasm. 
Others  develop  farther  and  farther  in  the  female  or  vegeta- 
tive direction ;  toward  a  decrease  in  activity ;  toward  a  stor- 
ing up  of  food  and  energy;  toward  a  great  preponderance 
of  trophoplasm. 

After  this  divergent  development  has  gone  on  for  a  time, 
in  each  case,  according  to  such  theories,  a  stage  of  too  great 
specialization  is  reached;  a  stage  of  lack  of  balance;  the 
male  has  developed  too  far  in  one  direction,  the  female  in 
the  other.  Life  can  therefore  no  longer  continue  vigorously 
and  normally;  the  chemical  changes  go  awry;  the  process 
that  we  call  aging  comes  on ;  reproduction  ceases ;  and  life 
must  end  unless  compensatory  changes  occur.  Such  com- 
pensatory changes  are  brought  about  by  a  reunion  of  the 
separated  male  and  female  substances  or  tendencies ;  the 
balance  is  thereupon  restored.  In  other  words,  rejuvenes- 
cence is  produced  by  mating ;  reproduction  can  now  go  on  as 
before ;  the  cycle  of  youthful  life  begins  anew. 

According  to  this  way  of  looking  at  the  matter,  there  is 


Nature  of  Sexual  Diversity  119 

an  actual  attraction  of  some  sort  between  the  two  kinds  of 
substance  when  separated,  and  this  it  is  which  causes  them 
to  unite.  In  the  case  of  entire  individuals,  and  particularly 
in  higher  organisms,  this  attraction  must  of  course  work 
through  many  indirect  means,  but  in  the  actual  union  of 
male  and  female  nuclei  or  of  chromosomes  (Figure  29)  it 
must  show  itself  in  some  direct  and  simple  manner,  as  when 
two  chemicals  unite  (compare  Doflein,  1911,  page  260). 
The  cause  of  the  union  is  the  diversity.  Thus,  according  to 
this  view,  the  attraction  of  the  sexes  has  its  basis  in  the 
foundations  of  life. 

To  be  consistent,  this  theory  has  to  maintain  that  in 
the  union  of  the  chromosomes  in  pairs,  which  appears  to 
be  the  elementary  act  in  mating,  there  is  this  same  diversity 
between  the  two  uniting  members,  and  that  this  is  the  cause 
of  their  union. 

Further,  it  would  appear  that  according  to  this  view, 
growing  old  is  fundamentally  a  different  thing  in  the  two 
sexes ;  in  the  female  it  would  be  the  consequence  of  an  excess 
in  the  vegetative  functions ;  in  the  male  a  consequence  of  ex- 
cess in  the  kinetic  functions.  This  consequence  I  have  not 
seen  drawn,  but  it  appears  an  unavoidable  one  if  the  theory 
is  held.  Doubtless  in  each  sex  it  could  be  held  that  some  other 
processes  essential  to  life  are  interfered  with  through  these 
changes,  in  such  a  way  as  to  give  similar  outward  signs  of 
age,  such  as  we  gee  in  the  two  sexes  of  higher  organisms. 

Let  us  now  turn  back  to  the  conjugation  of  Paramecium 
and  see  its  relation  to  such  ideas. 

First,  of  the  two  individuals  that  mate,  each  plays  the 
part  of  both  male  and  female.  Each  furnishes  a  smaller, 
active  half  nucleus,  which  moves  over  to  join  the  larger  in- 
active half  nucleus  of  the  other  individual;  this  active  half 
nucleus  evidently  takes  the  role  of  the  male  germ  cell.  But 


120         Life  and  Death,  Heredity  and  Evolution 

each  individual  of  the  pair  likewise  produces  a  larger  inac- 
tive half  nucleus,  which  remains  passive  and  is  sought  out 
by  the  smaller  one;  this  inactive  nucleus  plays  the  role 
of  the  egg  in  ordinary  fertilization.  If  there  is  a  funda- 
mental sex  difference  between  the  two  uniting  half  nuclei,  we 
must  call  the  active  one  male,  the  passive  one  female.  But 
the  two  individuals  that  unite  both  produce  male  and  female 
nuclei;  each  would  have  to  be  characterized  either  as  both 
male  and  female,  or  as  neutral.  There  appears  a  difficulty 
here,  for  why,  if  the  two  individuals  are  alike,  should  they 
be  drawn  together  and  mate?  According  to  the  theory  it  is 
unlikeness  of  sex  that  brings  about  mating. 

Attempts  have  been  made  to  show  that  the  two  individuals 
which  conjugate  really  are  of  preponderatingly  different 
sexes ;  that  one  exceeds  the  other  in  maleness,  the  other  in 
femaleness,  if  we  may  so  speak;  this  would  avoid  the  diffi- 
culty we  have  mentioned.  Calkins  (1902)  pointed  out  that 
often  only  one  of  the  two  individuals  that  have  mated  repro- 
duces freely,  the  other  reproducing  but  weakly  or  dying  with- 
out reproduction;  Miss  Cull  (1907)  showed  that  this  hap- 
pens in  a  great  number  of  cases.  The  individual  that  pro- 
duces young  after  mating  would  be  mainly  female,  while  the 
other  would  play  the  role  of  male.  Of  course  the  differentia- 
tion of  sexes  was  not  held  to  have  gone  far  in  such  a  case ; 
all  that  is  needed  is  that  one  should  be,  as  it  were,  more 
female  than  the  other;  then  the  diversity  would  operate  to 
produce  mating  and  consequent  rejuvenescence. 

This  idea  is  rendered  plausible  by  the  fact  that  in  some 
other  infusoria,  such  as  Vorticella  and  its  relatives,  there  is 
an  observable  difference  between  the  two  individuals  that 
conjugate.  One  of  the  two  mates  is  small  and  active,  taking 
the  usual  role  of  the  male,  while  the  other  is  larger  and 
remains  quiet,  taking  the  part  of  the  female  (Figures  33 


Sexual  Diversity  m  Protozoa 


121 


Figure  33.  Beginning  of  conjugation  in  Epistylis,  after  Wallengren, 
1899.  The  small  active  male  is  attaching  itself  to  the  side  of  the  large 
inactive  female. 


Figure  34.  Successive  steps  in  the  process  of  conjugation  in  Vorti- 
cella  nebulifera,  after  Maupas,  1889.  The  small  male  unites  with  the 
base  of  the  female;  its  protoplasm  entering  the  body  of  the  latter,  so 
that  finally  only  the  outer  layer  remains  outside  (at  F),  finally  drop- 
ping off. 


122          Life  and  Death,  Heredity  and  Evolution 

and  34).  In  these  cases  too  the  "male"  completely  unites 
with  the  female,  the  two  merging  into  one  (Figure  34),  just 
as  happens  with  the  two  germ  cells  of  higher  organisms; 
they  do  not  separate  and  continue  their  individual  lives,  as 
the  two  mates  do  in  Paramecium.  Each  of  the  mates  in 


Figure  35.  Diagrams  of  the  micronuclear  processes  in  conjugation, 
in  Paramecium  (P)  and  Vorticella  (V),  up  to  the  formation  of  the 
new  micronucleus  by  the  union  of  the  migratory  and  stationary  half 
nuclei.  In  each  case  the  original  micronuclei  of  the  two  mates  are 
shown  below,  and  successive  stages  and  divisions  are  shown  in  passing 
upward.  The  micronuclei  that  dissolve  and  disappear  are  shown  as 
clear  circles.  In  Vorticella  (V)  the  divisions  in  the  small  free  indi- 
vidual (microgamete)  are  shown  at  the  right;  those  of  the  large  stalked 
individual  (macrogamete)  at  the  left.  Based  on  diagrams  by  Maupas, 


Sexual  Diversity  in  Protozoa  123 

Vorticella  produces  just  before  the  fertilizing  process  two 
half  nuclei,  as  Paramecium  does,  but  in  the  large  "female" 
one  is  absorbed  and  disappears,  while  in  the  small  "male" 
likewise  one  is  absorbed  and  disappears.  The  remaining 
half  nucleus  of  the  "male"  then  passes  into  the  "female," 
and  unites  with  its  remaining  half  nucleus.  (See  the  dia- 
gram of  the  process  in  comparison  with  that  in  Paramecium, 
in  Figure  35). 


Figure  36.  Mating  in  the  mould,  Mucor,  after  De  Bary.  A  to  E, 
successive  stages.  The  large  black  body  at  E  formed  by  the  union  of 
the  ends  of  the  two  branches,  is  the  zygospore,  which  later  separates 
off,  and  produces  a  new  plant. 

In  Paramecium,  it  was  held  by  Cull  (1907)  that  there  is  a 
less  advanced  stage  of  a  similar  process ;  though  the  two  in- 
dividuals look  and  act  alike,  they  still  are  different,  since 
after  mating  one  reproduces  more  vigorously  than  the  other. 

Cases  are  known  in  which  the  individuals  of  a  species  are 
really  physiologically  diverse  as  to  sex,  although  in  appear- 
ance they  are  alike.  This  has  been  worked  out  fully  by 
Blakeslee  (1904)  in  the  common  moulds,  Mucor.  Here  the 


124«         Life  and  Death,  Heredity  and  Evolution 

filaments  produce  small  side  branches,  which  are  separated 
off  by  a  partition  as  club-shaped  germ  cells  or  reproductive 
bodies  (see  Figure  36).  These  reproductive  bodies  unite 
with  corresponding  bodies  from  another  individual  plant. 
By  test  it  is  found  that  these  reproductive  bodies  are  not 
indifferent  as  to  their  mates ;  with  reproductive  bodies  from 
certain  other  plants  they  will  not  unite.  Thorough  study 
shows  that  there  are  two  classes ;  that  those  of  one  class  will 
not  unite  with  those  of  the  same  class,  but  will  mate  with 
those  of  the  other  class.  The  characteristic  behavior  of  the 
two  sexes  occurs,  although  the  two  show  no  external  differ- 
ences (save  that  in  some  cases  the  "female"  plants  grow 
somewhat  more  luxuriantly  than  the  "male"  ones). 

Such  cases  show  that  the  physiological  distinction  of  two 
sexes  may  exist  without  any  structural  difference.  Is  this  so 
in  Paramecium?  The  case  here  turns  out  not  to  be  so  prob- 
able a  one  for  sex  difference  as  it  at  first  seemed.  It  is  true 
that  sometimes  after  mating  one  individual  reproduces  freely, 
while  the  other  does  not.  But  on  the  other  hand,  sometimes 
both  individuals  reproduce  freely ;  sometimes  both  reproduce 
feebly;  sometimes  neither  reproduces  at  all.  That  is,  there 
are  great  differences  among  the  individuals  that  have  con- 
jugated, in  respect  to  these  (and  other)  matters.  The  ques- 
tion therefore  reduces  itself  to  this :  Are  the  two  that  have 
mated  any  more  unlike  in  these  respects  than  are  any  two 
ordinary  individuals  that  have  not  mated  together?  For 
example,  if  one  multiplies  strongly,  is  its  mate  more  or 
less  likely  than  the  average  individual  to  multiply  weakly 
or  not  at  all? 

This  led  Lashley  and  myself  to  investigate  the  matter 
(Jennings  and  Lashley,  1913).  If  we  have  a  large  number 
of  cases  it  becomes  a  mathematical  problem  to  determine 
whether  the  two  members  of  a  pair  are  more  unlike  than 


Sexual  Diversity  m  Protozoa  125 

usual ;  there  are  definite  methods  for  solving  such  a  problem. 
Using  a  large  number  of  experiments,  we  found,  rather  un- 
expectedly, that  the  two  individuals  that  have  mated  are 
more  alike  in  all  these  respects,  not  less  alike.  If  one  mem- 
ber of  a  pair  dies,  its  mate  is  more  likely  to  die  than  are 
the  mates  of  individuals  that  live.  If  one  member  multi- 
plies feebly,  its  mate  is  likely  to  multiply  feebly,  too.  If  one 
member  multiplies  vigorously,  its  mate  is  likely  to  multiply 
vigorously  also.  All  these  relations  are  quite  the  opposite 
of  what  would  be  expected  if  there  are  pronounced  sex  dif- 
ferences between  the  members  of  pairs.  But  from  an  en- 
tirely different  point  of  view,  they  are  what  might  well 
be  expected.  The  two  mates  have  exchanged  parts,  so  that 
after  conjugation  each  is,  as  it  were,  half  composed  of 
material  from  the  other  (this  is  strictly  true  of  the  nuclei), 
so  that  it  is  natural  that  they  should  become  alike.  This 
brings  into  vie^  another  result  of  mating;  one  of  the  first 
importance.  Mating  causes  the  progeny  of  the  two  indi- 
viduals that  mate  to  be  alike.  This  point  we  shall  take  up 
later;  here  we  shall  look  farther  into  the  question  of  sex 
differences. 

We  have  therefore  no  positive  evidence  of  a  difference  of 
sex  between  the  two  members  of  a  pair  of  Paramecia.  On 
the  other  hand,  perhaps  it  can  hardly  be  maintained  that 
such  differences  are  disproved.  If  all  we  hold  is  that  some 
difference  in  degree  of  "maleness"  or  "femaleness"  is  what 
produces  mating,  it  becomes  extremely  difficult  to  test 
whether  this  holds  or  not.  In  this  connection  there  are  a 
number  of  facts,  some  of  them  very  curious,  that  require 
consideration. 

In  many  lower  organisms,  germ  cells  are  formed  by  the 
division  of  a  single  cell,  and  so  far  as  anyone  can  see,  these 
are  all  alike.  Further,  so  far  as  anyone  can  determine,  any 


126         Life  and  Death,  Heredity  and  Evolution 

two  of  these  can  mate, — just  as  apparently  any  two  indi- 
viduals can  mate  in  Paramecium.  This  is  particularly  the 
case  with  many  lower  plants  (Figure  37).  It  is  true  even 
in  many  of  the  moulds,  in  some  species  of  which,  as  we  have 
before  seen,  physiological  sex  differences  exist,  even  though 
there  is  no  visible  difference  between  the  sexes.  In  other 
species  of  moulds,  no  such  physiological  sex  difference  exists ; 
a  given  individual  can  mate  with  any  other  individual  of  the 
species. 


Figure  37.  Mating  of  similar  cells,  from  the  alga  Stephanosphaera ; 
successive  stages  from  left  to  right.  The  two  unite  to  form  a  single 
reproductive  body  or  zygote  (at  the  right).  After  Hieronymus,  from 
Doflein,  1911. 

From  such  cases,  common  in  lower  organisms,  it  is  natural 
to  draw  the  conclusion,  which  many  have  drawn,  that  in 
these  lowest  creatures  there  is  no  difference  of  sex ;  that  sex 
has  arisen  as  evolution  progressed;  that  therefore  sex  is 
not  coextensive  with  and  fundamental  to  life. 

On  the  other  hand,  some  have  held  that  the  very  fact  that 
two  unite  is  sufficient  proof  that  they  are  sexually  diverse; 
this  for  example  is  the  view  taken  by  Minchin  (1912)  in 
his  Introduction  to  the  Study  of  the  Protozoa  and  by  Coul- 
ter in  his  interesting  work  on  the  Development  of  Sex  in 
Plants  (1914).2  If  this  argument  is  advanced  as  one  of 

'See  Minchin,  p.  160:  "The  fact  that  gametes  and  pronuclei  tend  to 
unite  proves  that  in  all  cases  there  must  be  intrinsic  differences  be- 
tween them  which  stimulate  them  to  do  so."  Coulter  says  on  page  26: 

"The  gametes  are  alike  in  appearance,  but  that  they  are  not  alike  in 
fact  is  evidenced  by  their  pairing  and  mutual  attraction."  A  similar 
idea  is  expressed  by  Collin  (1909):  "But  this  invisible  difference  be- 


Nature  of  Sexual  Diversity  127 

general  validity,  it  of  course  requires  one  to  hold  that  the 
two  chromosomes  which  unite  at  their  mating  within  the  cell 
are  sexually  diverse. 

But  it  hardly  appears  possible  to  argue  on  general 
physical  grounds  that  attraction  and  union  imply  diversity ; 
two  masses  of  substance  of  identical  physical  and  chemical 
constitution  show  the  attraction  called  gravitation;  they 
tend  to  come  together  and  unite.  Adhesion  occurs  between 
bodies  of  like  character,  and  in  general  it  is  not  clear  that 
all  attraction  of  bodies  must  be  due  to  chemical  or  physical 
diversities  between  them.3  It  appears  therefore  that  we 
should  look  for  further  evidence  before  holding  that  union 
is  itself  an  evidence  of  sex  diversity.  If  the  two  things 
that  unite  are  characteristically  diverse,  the  difference  must 
show  itself  in  other  ways,  so  that  we  must  rely  on  other 
tests  to  determine  whether  the  diversity  exists.  Is  there 
any  evidence  that  the  two  chromosomes  that  unite  in  mat- 
ing are  sexually  diverse? 

In  Vorticella  and  its  relatives,  as  we  have  seen,  the  two 
individuals  that  mate  are  diverse  (Figures  33  and  34).  But 
curiously,  these  two — the  "male"  and  "female" — are  formed, 
in  some  species  at  least,  by  the  division  of  a  single  ordinary 
fixed  individual  into  a  male  and  a  female  individual.  The 
ordinary  fixed  individuals  before  this  division  do  not  mate, — 

tween  the  two  mating  cells  is  necessary  a  priori,  and  theoretically  re- 
quired. For  it  would  not  be  intelligible  that  two  identical  cells  coming 
in  contact  should  affect  one  another,  stimulate  one  another  out  of  the 
state  of  repose  and  cause  the  complicated  sexual  processes  to  begin" 
(p.  376). 

1  It  is  doubtless  possible  that  all  physical  attraction  between  bodies  is 
due  to  their  make-up  of  the  two  opposite  kinds  of  electrons,  so  that 
in  this  sense  attraction  may  be  due  to  diversity.  But  this  is  diversity 
within  the  bodies:  the  two  masses,  as  such,  may  have  the  same  consti- 
tution (including  these  diversities)  and  still  attract.  That  is,  the  attrac- 
tion of  the  two  cells  or  individuals  that  mate  does  not  a  priori  imply 
any  greater  diversity  between  them  than  that  between  two  leaden  bullets 
that  gravitate  toward  one  another. 


128         Life  and  Death,  Heredity  and  Evolution 

neither  with  one  another  nor  with  any  males  that  may  be 
present.  So  the  ordinary  individual  is  neither  male  nor 
female;  it  is  apparently  non-sexual  or  neutral.  After  it 
has  divided  to  produce  male  and  female,  the  female  resembles 
externally  the  neutral  individual,  but  differs  from  it  in 
that  it  may  mate.  If  it  has  no  opportunity  to  mate,  it 
may  return  to  the  neutral  condition,  and  may  then  again 
divide  to  produce  males  and  females.  These  facts  have 
been  worked  out  in  Opercularia  by  Enriques  (1907).  In 
some  other  Vorticellidae  a  single  ordinary  individual  may 
divide  into  four  or  eight  males.  It  is  asserted  that  some- 
times the  small  motile  "males"  mate  together;  and  that 
similarly  sometimes  two  fixed  "females"  mate;  but  these 
things  are  uncertain.  There  is  still  much  to  be  learned 
as  to  sex  diversity  in  this  most  interesting  group  of  the 
Vorticellida. 

In  some  infusoria  the  two  individuals  that  mate  are  alike 
before  mating,  but  become  changed  and  diverse  during  the 
mating  process.  This  is  inevitable  in  some  infusoria,  since 
the  organisms  are  unsymmetrical,  and  if  their  mouth  sur- 
faces are  to  be  brought  together,  either  the  two  must  take 
different  positions,  or  one  or  both  must  become  altered 
in  structure.  This  will  be  evident  from  Figure  38. 

Enriques  (1908)  studied  this  matter  thoroughly  in 
Chilodon.  In  this  animal  the  mouth  lies  to  the  left  on  the 
ventral  surface,  so  that  when  the  two  individuals  are  placed 
side  by  side,  the  mouth  of  the  right-hand  individual  lies  at 
the  surface  of  contact,  but  the  mouth  of  the  left-hand  in- 
dividual is  directed  away  from  the  surface  of  contact  (Fig- 
ure 38).  But  in  the  early  stages  of  the  process  of  mating, 
the  mouth  of  this  left-hand  individual  moves  along  the  ventral 
surface  over  to  the  right, — so  as  to  meet  the  mouth  of  the 
other  individual  which  does  not  change  its  position  (Figure 
39).  The  anterior  end  of  the  left  individual  likewise  be- 


Nature  of  Sexual  Diversity 


129 


Figure  38.  Chilodon.  Two  individuals  placed  side  by  side,  showing 
that  the  mouths,  lying  at  the  left  side,  will  not  come  in  contact  in 
conjugation  without  some  alteration  of  position.  Based  on  a  figure 
by  Enriques,  1908. 


Figure  39.  Conjugation  of  Chilodon.  The  mouth  of  the  left-hand 
individual  has  moved  over  to  the  right,  to  meet  that  of  the  right-hand 
individual.  After  Enriques,  1908  (but  reversed  in  position). 

comes  bent,  in  such  a  way  as  to  shorten  the  animal,  so  that 
when  the  two  members  of  a  pair  are  measured,  the  left 
member  is  shorter  than  the  right-hand  one.  The  result  of 
all  this  is  that  the  two  mates  have  become  diverse.  Enriques 
holds  this  to  be  the  first  stage  in  the  production  of  sex  differ- 
ences; that  the  right-hand  individual  corresponds  to  a  fe- 
male, the  left-hand  one  to  a  male.  He  believes  that  in  other 
species  the  difference  thus  produced  in  the  mating  process 


130         Life  and  Death,  Heredity  and  Evolution 

lasts  and  is  inherited,  so  that  the  two  sexes  are  distinguish- 
able even  before  mating  occurs.  There  is  no  evidence  that 
such  inheritance  occurs.  This  notion  of  the  origin  of  sex 
of  course  implies  that  there  is  no  underlying  general 
physiological  difference  that  makes  sex ;  sex  would  be  a  mere 
matter  of  the  external  differences.  If  we  believe  that  there 
is  such  a  general  physiological  difference,  then  we  could 
hardly  hold  that  these  differences  produced  during  mating 
are  sex  diversities,  unless  we  believe  that  they  are  guided 
by  and  coincide  with  the  previously  existing  physiological 
differences.  It  appears  probable  that  the  diversities  be- 
tween the  two  mating  individuals  in  Chilodon  are  mere 
transient  alterations,  with  no  lasting  consequences. 

So  far  we  have  dealt  with  the  differences,  or  lack  of 
differences,  between  the  two  individuals  that  mate.  But  the 
sex  diversities  reach  to  the  more  intimate  structures ;  to  the 
nuclei.  In  Paramecium  (Figure  40),  each  individual  pro- 
duces a  larger  half  nucleus  that  is  passive,  like  the  female; 
a  smaller  half  nucleus,  which  is  active,  seeking  out  and 
uniting  with  the  passive  half  nucleus  of  the  other  mate, 
and  thus  playing  the  part  of  the  male.  What  is  the  differ- 
ence between  these  two  half  nuclei  produced  by  a  single 
individual, — the  "male"  and  "female"  half  nuclei?  Here  we 
have  the  problem  of  the  nature  of  sex  diversity  brought  to 
a  point ;  if  we  could  answer  this  question  for  these  two 
half  nuclei,  we  should  know  the  nature  of  sex. 

The  differences  that  we  observe  are  the  following  (see 
Figure  40  and  Figure  49,  H).  The  "male"  half  nucelus  is 
a  little  smaller;  it  is  usually  nearer  to  the  surface  of  union 
of  the  two  mates ;  it  separates  from  the  "female"  half  nucleus 
of  the  same  individual ;  it  moves  toward  and  across  the  sur- 
face of  union ;  it  refuses  to  unite  with  the  other  "male"  half 
nucleus  which  it  meets  and  passes  on  its  way;  but  it  does 


Nature  of  Sexual  Diversity 


131 


Figure  40.  The  exchange  of  the  half  nuclei  in  Paramecium  caudatum, 
after  Maupas,  1889.  The  spindle-shaped  bodies  are  the  half  nuclei. 
A.  The  micronucleus  of  each  member  of  the  pair  is  dividing  into  two 
halves,  which  are  still  connected  by  a  band.  The  upper  half  in  each 
case  is  the  "male"  or  "migratory"  half  nucleus.  B.  The  two  migratory 
half  nuclei  are  passing  one  another  at  the  surface  of  separation  of  the 
two  individuals.  (Each  is  still  connected  by  a  long  band  with  the 
other  half  (stationary)  of  the  nucleus  from  which  it  came.)  C.  The 
two  migratory  half  nuclei  are  close  together,  but  have  now  completely 
lost  their  connection  with  the  stationary  halves.  D.  The  migratory 
halves  have  nearly  separated,  each  passing  into  the  body  of  the  opposite 
individual.  E.  The  migratory  and  stationary  half  nuclei  have  come  in 
contact.  F.  The  migratory  and  stationary  half  nuclei  have  almost 
united. 


132          Life  and  Death,  Heredity  and  Evolution 

unite  with  the  "female"  half  nucleus  of  the  other  individual. 
The  "female"  half  nucleus  is  a  little  larger;  it  simply  re- 
mains quiet,  finally  uniting  with  the  "male"  half  nucleus  which 
migrates  to  it. 

The  problem  of  sex  comes,  in  perhaps  its  sharpest  form 
in  the  question:  What  makes  the  two  "male"  half  nuclei 
refuse  to  unite  when  they  come  in  contact,  while  the  "male" 
and  "female"  do  unite? 

One  answer  to  all  these  questions  is  that  given  by  Maupas 
(1889).  According  to  this,  there  is  no  physiological  differ- 
ence between  the  two  half  nuclei ;  their  difference  of  behavior 
is  a  mere  result  of  their  accidental  difference  of  position. 
The  migratory  half  does  not  move  through  any  peculiarity 
of  its  own ;  the  fact  is  merely  that  the  half  nearest  the  surface 
of  separation  is  seized  by  the  movements  of  the  surrounding 
cytoplasm  and  carried  over  into  the  other  mate.  The  fact 
that  the  two  migratory  halves  do  not  unite  is  again  merely 
due  to  their  being  pulled  along  by  the  cytoplasm.  Save 
for  this  activity  of  the  cytoplasm,  the  two  migratory  half 
nuclei  could  just  as  well  unite  with  each  other  as  with  the 
inactive  half  nuclei.  According  to  this  way  of  looking  at 
the  matter,  there  is  no  sex  difference  between  these  two 
half  nuclei;  indeed,  sex  has  no  meaning,  save  as  a  name 
for  certain  external  peculiarities. 

This  is  one  possible  opinion  on  the  matter ;  it  rests  upon, 
or  results  in,  the  general  view  that  there  is  no  general 
underlying  peculiarity  that  constitutes  sex  diversity;  and 
that  sexuality  is  not  a  general  characteristic  of  living  things. 

The  other  possible  view  is  that  the  difference  in  appearance 
and  behavior  of  the  two  half-nuclei  is  a  consequence  of  an 
underlying  physiological  difference  that  makes  sex.  In  sup- 
port of  this  view  there  are  urged  certain  facts  observed  by  a 
number  of  later  investigators.  One  is  that  it  is  not  true 


Nature  of  Sexual  Diversity  133 

that  the  half  nucleus  nearest  the  surface  of  separation  of 
the  two  mates  is  always  the  one  that  moves  over  into  the 
other  mate;  on  the  contrary,  sometimes  one  of  the  nuclei 
farther  from  this  surface  is  the  one  that  thus  becomes  the 
migratory  Jialf  nucleus.1  Further,  the  actual  smaller  size  of 
the  migratory  half  nucleus,  observed  in  Didinium  by  Prandtl 
(1906),  and  in  Paramecium  by  Calkins  and  Cull  (1907),  is 
held  to  imply  an  intrinsic  difference  between  the  two.  Ac- 
cording to  this  way  of  looking  at  the  matter,  one  of  the  half 
nuclei  is  male,  the  other  female,  in  virtue  of  their  diverse 
chemical  make-up.  Such  a  view  goes  with  the  general  theory 
that  sex  in  a  matter  of  fundamental  physiological  diversity, 
not  a  mere  name  for  certain  external  peculiarities. 

Keeping  these  two  contrasted  opinions  in  mind,  certain 
facts  as  to  what  occurs  in  these  organisms  are  of  much 
interest.  The  theory  of  the  need  for  periodic  unions;  the 
theory  of  rejuvenescence  through  such  unions,  is  based, 
as  we  have  seen,  on  the  notion  that  the  two  nuclei  have 
been  developing  in  opposite  directions, — one  toward  "male- 
ness,"  the  other  toward  "femaleness,"  till  an  unbalanced  con- 
dition is  reached;  union  is  then  required  to  restore  the  bal- 
ance. 

The  situation  in  Paramecium  and  other  infusoria  seems 
almost  to  reduce  this  idea  to  an  absurdity  as  an  explanation 
of  a  necessity  for  periodic  mating.  For  here  the  two  nuclei 
that  are  assumed  to  have  been  developing  in  opposite  direc- 
tions,— the  "male"  and  "female"  half  nuclei, — have  in  fact 
been  developing  continuously  together,  in  the  same  micro- 
nucleus  of  the  same  individual !  It  is  only  immediately  be- 
fore the  union  of  "male"  and  "female"  parts  in  the  mating 
that  the  united  "male"  and  "female"  parts  have  separated. 

Calkins  and  Cull,  1907,  p.  393;  Prandtl,  1906,  p.  246;  Collin,  1909, 
p.  359,  etc. 


134«         Life  and  Death,  Heredity  and  Evolution 

There  is  no  point  to  the  male  element's  separating  from 
the  female  element,  to  immediately  unite  with  another  female 
element,  if  the  whole  rationale  of  the  process  is  merely  to 
bring  maleness  and  femaleness  together;  they  needed  but 
to  remain  where  they  were ! 

From  the  fact  that  "male"  and  "female"  of  one  individual 
separate,  and  immediately  unite  with  "female"  and  "male" 
respectively  of  another  individual,  it  might  seem  that  the 
essential  point  is  not  the  mere  union  of  male  and  female, 
but  the  union  of  parts  of  diverse  individuals,  and  many 
have  held  this  to  be  the  case.  But  here  again  we  meet  a 
set  of  extraordinary  facts  which  make  this  way  of  looking 
at  the  matter  as  difficult  as  the  other.  In  many  organisms 
the  unions  that  occur  are  between  half  nuclei  of  the  same 
individual,  or  even  of  the  same  cell;  this  is  the  process  that 
is  becoming  familiar  under  the  name  autogamy. 

A  typical  striking  case  of  this  union  of  half  nuclei  from 
the  same  cell  is  seen  in  the  small  parasitic  flagellate  in- 


Figure  41.  Mating  of  two  halves  of  the  nucleus  of  same  cell  (auto- 
gamy) in  the  cyst  of  Trichomastix.  A,  division  of  the  nucleus;  B,  the 
two  nuclei  separated  by  a  large  mass  of  reserve  food  matter;  C,  first 
"reduction"  division  of  each  nucleus;  D,  second  reduction  division;  E, 
the  two  nuclei  approach  each  other;  F,  they  unite  into  one.  After 
Prowazek  from  Hartmann,  1909. 


Mating  of  Similar  Parts  135 

fusorian  Trichomastix,  found  in  lizards.  The  flagellate 
forms  a  small  round  cyst  (Figure  41),  in  which  the  single 
nucleus  divides  into  two  (A),  the  two  nuclei  coming  to  lie 
one  on  each  side  of  a  large  mass  of  reserve  nutritive  material. 
Then  each  nucleus  divides  twice  unequally,  giving  off  two 
small  nuclei  (Figure  41,  C  and  D).  [These  two  small  nuclei, 
commonly  known  as  reduction  nuclei,  are  absorbed  and  dis- 
appear. The  remaining  two  nuclei,  which  we  can  now,  for 
reasons  to  be  clearly  seen  later,  call  half  nuclei,  move 
toward  each  other  (E),  and  finally  unite  (F).  The  process 
of  mating  or  of  fertilization  is  now  finished. 

Here  again  we  have  a  case  in  which  the  two  half  nuclei 
that  unite  have  been  developing  continuously  together,  in 
a  single  nucleus ;  they  separate,  then  ( after  two  more  divi- 
sions) reunite.  And  in  this  case,  it  is  the  same  two  nuclei 
that  have  separated  that  come  together  again, — save  for  the 
fact  that  each  has  divided  off  two  small  nuclei. 

The  same  sort  of  thing  occurs  in  many  lower  organisms. 


Figure  42.  Two  methods  of  conjugation  in  Spirogyra,  after  Walton, 
1915.  A,  conjugation  between  adjoining  cells  of  the  same  filament. 
The  contents  of  the  cell  to  the  right  have  passed  into  the  other  cell  and 
united  with  its  contents.  B,  conjugation  between  the  cells  of  different 
filaments.  From  the  filament  above,  the  cell  contents  have  passed  into 
the  cells  of  the  filament  below. 


136          Life  and  Death,  Heredity  and  Evolution 

Sometimes,  as  in  the  case  just  described,  it  is  two  nuclei  of 
the  same  cell  that  reunite.  In  other  cases  it  is  half  nuclei 
from  two  adjoining  cells  of  the  same  organism;  this  hap- 
pens in  the  alga  Spirogyra  (Figure  42).  Or  again,  two 
branches  of  the  same  plant  may  unite,  with  union  of  nuclei 
(Figure  43).  Or  again,  two  cells  that  have  been  formed  by 
the  division  of  a  single  cell  may  mate ;  this  happens  in  Para- 
mecium  aurelia  at  times,  as  well  as  in  other  infusoria,  and 
in  algas.  We  find  every  possible  transition,  from  the  union 
of  two  half  nuclei  of  a  single  cell  (Figure  41)  to  the  fertili- 
zation of  one  individual  by  another  quite  unrelated  to  it. 

In  all  these  cases  in  which  the  two  half  nuclei  that  unite 
have  been  developing  together  in  a  single  nucleus,  evidently 


Figure  43.  Process  of  conjugation  of  two  branches  of  the  same 
plant,  in  the  mould  Zygorhynchus.  A  to  H,  successive  stages,  leading 
to  the  formation  of  the  dark  zygospore  in  H.  After  Blakeslee,  1913. 


Mating  of  Similar  Parts  137 

we  cannot  explain  the  process  as  due  to  the  gradually  diver- 
gent development  of  two  nuclei,  one  in  the  male  direction, 
the  other  in  the  female  direction,  till  they  have  become  so 
diverse  as  to  be  unbalanced,  and  so  to  require  reunion.  And 
there  is  no  ground  on  this  basis  for  any  rejuvenescence  to  be 
produced  by  the  union ;  for  the  male  and  female  parts  that 
become  united  were  already  in  union  before  the  separation 
and  reunion  occurred. 

A  modified  form  of  this  notion  is  held  by  some  students, — 
a  form  so  modified  as  to  be  almost  if  not  quite  empty.  In 
such  a  case  of  reunion  of  two  half  nuclei  of  a  single  cell 
as  we  see  in  Figure  41,  as  well  as  in  all  other  cases,  it  is 
maintained  that  the  two  half  nuclei  have  become  diverse,  in 
the  divisions  that  have  just  occurred, — one  retaining  more 
of  the  kinetic  or  male  characteristics,  the  other  more  of  the 
vegetative  or  female  characteristics ;  and  that  this  is  the 
reason  why  the  two  now  unite.  That  is,  after  the  two  half 
nuclei  have  separated,  this  theory  if  correct  gives  a  ground 
for  their  reuniting.  But  it  gives  no  ground  at  all  for  the 
fact  that  the  organism  periodically  goes  through  this  whole 
process,  of  separating  off  two  half  nuclei,  which  then  again 
unite, — since  what  is  accomplished  by  their  union  was  al- 
ready existent  before  the  process  occurred.  And  it  gives 
no  ground  for  expecting  any  rejuvenescence  or  other  marked 
physiological  result  from  mating.  That  is,  it  gives  no  ex- 
planation for  periodic  mating  such  as  is  given  for  periodic 
taking  of  food,  when  we  show  that  it  is  the  taking  of  food 
that  makes  possible  the  activities  and  growth  of  organisms. 
It  is  therefore  not  surprising  that  in  a  paper  maintaining 
this  theory,  Hartmann  (1909)  concludes  with  the  statement 
that  he  thinks  it  most  improbable  that  this  will  turn  out  to 
be  the  full  explanation  of  the  matter. 

On  the  whole,  it  appears  that  in  Paramecium  and  many 


138          Life  and  Death,  Heredity  and  Evolution 

other  infusoria,  both  mates  of  a  pair  play  the  part  of  both 
"male"  and  "female,"  so  that  diversity  of  sex  cannot  be 
given  as  ground  for  the  union  of  the  two  individuals.  Fur- 
ther, there  is  no  clear  evidence  that  the  two  uniting  half 
nuclei  are  diverse  in  any  generally  characteristic  way ;  indeed 
on  the  whole  the  facts  perhaps  agree  best  with  the  view  that 
there  is  no  underlying  sexual  difference  in  the  uniting  half 
nuclei;  that  indeed  in  many  of  these  lower  organisms  there 
is  no  such  thing  as  sex  diversity. 

And  this  too  appears  the  natural  conclusion  with  rela- 
tion to  that  ultimate  act  of  mating,  the  union  of  the  chromo- 
somes. In  many  organisms  this  does  not  occur  at  once  after 
the  mating  of  the  individuals  or  the  germ  cells  (though  in 
some,  particularly  in  the  flies,  it  does).  Usually  after  the 
germ  cells  unite,  the  chromosomes  remain  without  mating  for 
many  cell  generations,  through  which  the  fertilized  egg  de- 
velops into  the  body  of  a  new  individual.  It  is  only  when 
the  germ  cells  of  this  new  individual  are  ripening  for  their 
next  mating  that  the  chromosomes  within  each  cell  mate. 
At  this  time  they  form  a  set  of  structures  that  in  some  or- 
ganisms show  many  diversities  of  size  and  form  (Figure  29). 
But  the  mating  is  not  between  the  most  diverse  individuals. 
On  the  contrary,  we  find  as  a  rule  that  each  chromosome  has 
found  as  its  mate  one  precisely  like  itself, — so  that  the 
group  was  really  composed  of  sets  of  two  individuals  of  the 
same  size  and  form,  and  it  is  these  two  that  mate.  Excep- 
tionally two  chromosomes  of  diverse  size  or  form  may  mate, 
but  this  is  only  when  a  similar  mate  is  lacking,  and  there  is 
evidence  that  such  mating  between  chromosomes  of  diverse 
size  or  form  is  not  so  intimate  as  that  between  chromosomes 
of  similar  structure.  All  the  indications  are  that  in  this  ul- 
timate act  of  mating  the  union  is  Between  structures  that  are 


Nature  of  Sexual  Diversity  139 

alike,  not  structures  that  are  characteristically  diverse.4 
The  theory  of  two  essentially  diverse  substances,  male  and 
female,  cannot  be  applied  in  any  form  to  the  mating  of  the 
chromosomes. 

But  if  similarity  between  the  parts  that  mate  is  the  orig- 
inal and  elementary  condition,  how  does  it  happen  that  in 
other  organisms,  and  indeed  in  most,  we  do  find  a  diversity  of 
sex?  Why  do  we  find  that  as  a  rule  in  higher  organisms  a 
small  germ  cell  unites  with  a  larger  one? 

This  has  often  been  conceived  as  a  special  case  of  division 
of  labor.  It  is  necessary  for  movement  to  take  place  in 
order  that  union  shall  occur ;  it  is  also  necessary  that  there 
shall  be  a  certain  amount  of  food  or  stored  up  energy  for 
the  beginning  of  development  in  the  new  organism.  Hence 
it  appears  conceivable  that  variations  should  arise  among 
the  originally  equivalent  cells, — such  that  one  set  would 
become  more  active,  while  the  other  would  store  up  more 
food.  This  of  course  could  not  occur  without  a  correspond- 
ing change  in  the  underlying  chemical  processes,  but  the 
difference  in  sex  would  according  to  this  view  not  be  coex- 
tensive with  life.  It  would  be  a  difference  that  has  arisen 
in  evolution,  just  as  the  difference  between  two  races  of 
Difflugia  has  arisen.  In  Paramecium  and  related  organisms, 
according  to  this  view,  the  difference  has  not  arisen. 

What  chiefly  raise  doubt  as  to  the  correctness  of  this  idea 
are  the  cases  in  which  there  is  a  physiological  sex  diversity 
where  no  structural  diversity  can  be  seen.  But  there  exist 

4  The  chromosomes  are  structures  that  perpetuate  themselves  from 
generation  to  generation,  by  division,  as  Protozoa  do.  It  is  not  difficult 
to  so  arrange  breeding  experiments  that  two  offspring  of  the  same 
chromosome  shall  in  a  later  generation  be  found  in  different  sexes  and 
mate  together,  just  as  we  may  cause  two  offspring  of  the  same  Para- 
mecium to  breed  together.  This  again  would  be  difficult  to  reconcile 
with  the  idea  that  the  two  mating  chromosomes  must  be  sexually  di- 
verse. 


140         Life  and  Death,  Heredity  and  Evolution 

equally  cases  in  which  by  any  physiological  test  that  can 
be  applied,  there  is  no  diversity  of  sex. 

The  only  doctrine  of  sex  diversity  that  appears  to  offer 
possibilities  of  general  application  is  one  that  holds,  not 
that  there  is  always  a  definite  positive  characteristic  for 
each  sex,  but  only  that  there  is  some  relative  difference  be- 
tween the  two  mates,  one  of  them  being  somewhat  more 
developed  in  a  certain  direction  than  the  other.  It  would 
not  be  inconsistent,  on  this  view,  if  a  given  individual  or 
cell  or  chromosome  could  play  the  part  of  male  in  some 
matings ;  of  female  in  others, — depending  on  the  relative  de- 
velopment of  the  two  mates.  But  such  a  doctrine  is  cer- 
tainly not  established;  and  for  the  mating  of  the  chromo- 
somes it  has  not  a  particle  of  evidence. 

It  is  most  important  not  to  commit  the  error  common  in 
evolutionary  thought,  of  identifying  the  question  of  the 
primitive  nature  of  a  phenomenon  with  that  of  its  physio- 
logical significance  where  it  is  developed  in  a  marked  degree. 
The  question  whether  all  organisms  have  sex  has  little  to  do 
with  the  question  as  to  what  the  nature  and  results  of  sex 
are,  in  organisms  that  are  divided  into  two  sexes.  In  such 
organisms — in  man  for  example — the  fundamental  chemical 
processes  that  make  up  life  are  in  the  two  sexes  diverse  in 
every  cell  of  the  body,  for  the  nuclei  on  which  these  processes 
depend  are  diverse.  There  is  no  ground  therefore  for  sup- 
posing that  the  characteristic  differences  in  the  activities  of 
the  two  sexes  are  mere  matters  of  different  education  or  en- 
vironment. 


VI 


What  are  the  Results  of  Mating?     Rejuvenescence  and 
Mating.    Heredity  and  Variation,  and  Mating. 

WE  have  so  far  tried  to  judge  of  the  nature  and  effects 
of  mating  chiefly  by  examining  the  organisms  or 
parts  that  unite,  and  the  processes  that  occur  in  union. 
Now  we  turn  to  the  study  by  observation  and  experiment  of 
the  effects  of  mating.  Does  mating  actually  cause  re- 
juvenescence, in  the  sense  pf  an  increased  vigor  and  vital- 
ity? The  chief  definite  criterion  for  such  increased  vigor 
that  has  been  proposed  is,  that  after  mating,  reproduction 
should  take  place  more  energetically  than  before.  In  the 
Protozoa,  for  example,  old  age  is  held  to  show  itself  by  a 
decrease  in  the  rate  of  multiplication;  rejuvenescence  by 
an  increase.  This  is  the  theory  which  Calkins  in  recent 
times  has  made  peculiarly  his  own.  "As  with  the  metazoon 
so  with  the  aggregate  of  Protozoa  cells,  we  note  a  period 
of  youth  characterized  by  active  cell  proliferation;  this  in 
both  groups  of  organisms  is  followed  by  the  gradual  loss 
of  the  division  energy  accompanied  by  morphological 
changes  in  type  of  the  cells  preliminary  to  conjugation  and 
fertilization  and  to  the  renewal  of  vitality  by  this  means" 
(Calkins,  1906,  p.  232).  This  appears  to  be  what  is  com- 
monly understood  by  the  theory  of  rejuvenescence. 

How  can  we  find  out  whether  this  theory  is  correct  ?    Evi- 
dently the  direct  method  is.  to  take  a  stock  of  infusoria,  such 
as  Paramecium,  and  allow  it  to  multiply  till  it  gets  to  the 
141 


148         Life  and  Death,  Heredity  and  Evolution 

stage  in  which  it  is  ready  to  conjugate.  Then  permit  part 
to  conjugate,  while  the  rest  are  not  allowed  to  do  so,  and 
compare  the  rate  of  reproduction  in  the  two  sets,  under 
identical  conditions.  Will  those  that  have  been  allowed  to 
conjugate  reproduce  more  rapidly  than  the  others? 

This  experiment  was  first  performed  by  Richard  Hertwig 
(1889).  At  the  beginning  of  conjugation  in  Paramecium 
he  separated  some  of  the  pairs  before  the  process  had  been 
accomplished,  while  others  he  allowed  to  complete  the  mat- 
ing processes ;  he  then  compared  the  rate  of  fission  in  the 
two  sets.  Calkins  in  1901  similarly  compared  the  rate  of 
fission  in  a  single  line  derived  from  an  ex-con  jugant  with 
the  non-con  jugant  stock  from  which  it  came  (the  experi- 
ment is  described  in  full  by  Calkins  and  Gregory,  1913). 
I  carried  out  the  same  experiment  on  a  very  large  scale 
in  1913,  employing  large  numbers  of  lines  of  both  non-con- 
jugant  and  con  jugant  origin,  and  repeating  the  experiment 
many  times.  Mast  (1917)  has  recently  repeated  the  ex- 
periment with  a  number  of  lines  of  Didinium,  and  Calkins 
(1919)  has  still  more  recently  carried  it  out  with  Uroleptus. 

Furthermore,  Maupas  (1888  and  1889)  made  extensive 
and  thorough  researches  on  the  same  question  in  many  kinds 
of  infusoria,  by  somewhat  different  methods.  He  did  not 
separate  pairs  that  were  beginning  to  mate,  as  was  done 
in  the  other  experiments,  but  compared  the  rate  of  fission 
soon  after  conjugation  with  the  rate  after  many  genera- 
tions had  elapsed  since  conjugation. 

All  these  experiments  except  those  which  have  been  re- 
ported by  Calkins,  gave  concordant  results,  unfavorable  to 
the  idea  that  conjugation  increases  the  rate  of  fission.  Mau- 
pas found  that  there  was  in  none  of  the  cases  he  studied  an  in- 
crease of  fission  rate  after  conjugation;  nor  a  decrease  as 


Conjugation  and  Hate  of  Reproduction  143 

the  time  elapsed  since  conjugation  became  greater.  Hertwig 
found  that  the  descendants  of  those  that  had  conjugated 
actually  multiplied  more  slowly  than  the  descendants  of  their 
companions  that  were  not  allowed  to  conjugate.  Mast  found 
that  in  Didinium  the  rate  of  fission  after  conjugation  is  a 
little  slower  for  a  short  time ;  but  soon  the  rate  becomes  equal 
in  those  that  have  and  those  that  have  not  lately  conjugated. 
In  my  own  extensive  experiments,  with  large  numbers  of 
lines  in  both  classes,  the  average  fission  rate  was  uniformly 
less  in  the  lines  that  had  recently  conjugated  than  in  those 
that  had  not.  Conjugation,  as  we  shall  see,  caused  much 
variation  between  the  diverse  lines,  in  fission  rate  as  well  as  in 
other  hereditary  characters.  At  the  upper  extreme  of  the 
variation  sometimes  a  conjugant  line  exceeded  in  its  fission 
rate  the  non-con jugant  lines.  But  in  most  of  the  lines  the 
rate  after  conjugation  was  decidedly  less  than  before;  uni- 
formly the  average  of  the  conjugant  lines  was  much  below 
that  of  the  non-conjugant  ones.1 

In  Calkins'  experiments,  on  the  other  hand,  the  lines  which 
had  conjugated  showed  a  more  rapid  fission  rate  than  those 
which  had  not.  In  his  experiment  on  Paramecium  in  1901,  in 
which  a  single  line  derived  from  an  ex-con  jugant  was  com- 
pared with  the  non-conjugant  stock  from  which  it  had  come, 
the  ex-con  jugant  line  divided  376  times  in  nine  months, 
while  the  non-conjugant  lines  produced  but  277  generations. 
It  may  be  noted  that  another  conjugant  line,  derived  from 
the  mate  of  the  line  that  produced  376  generations,  died  out 
completely  after  the  production  of  but  11  generations.  This 
illustrates  the  fact  shown  on  a  large  scale  in  my  own  ex- 
periments, that  conjugation  brings  about  great  variation  in 

irThe  actual  comparative  rates  in  certain  cases  will  be  given  later 
(page  158). 


144          Life  and  Death,  Heredity  and  Evolution 

vitality  and  reproductive  power.  The  single  line  retained  by 
Calkins  was  clearly  one  of  the  rare  extreme  variants,  with  a 
high  fission  rate. 

In  Calkins'  recent  work  on  Uroleptus.2  it  is  set  forth  that 
in  this  organism  the  effect  of  conjugation  is  uniformly  to  in- 
crease the  rate  of  fission.  The  fission  rate,  however,  is  given 
in  terms  of  averages  of  sets  of  five  lines ;  such  averages  were 
always  higher  for  the  sets  that  had  recently  conjugated  than 
for  those  that  had  not.  But  it  is  not  possible  to  discover 
from  them  whether  the  fission  rate  was  higher  in  all  the  lines 
of  conjugants  or  whether  as  in  my  own  experiments  the  rate 
was  lower  in  some  of  the  conjugant  lines,  higher  in  the  oth- 
ers. Obviously  it  was  higher  in  the  majority  at  least  of  the 
conjugated  lines. 

The  fact  that  some  of  the  lines  derived  from  ex-con jugants 
may  exceed  in  their  fission  rate  the  non-conjugant  stocks, 
whether  this  is  rare,  as  in  Paramecium  and  apparently  in 
most  other  infusoria,  or  common,  as  in  Uroleptus,  is  unques- 
tionably a  fact  of  great  significance ;  this  renders  it  possible 
to  still  maintain,  as  Calkins  does,  that  mating  at  least  some- 
times produces  rejuvenescence,  in  the  sense  of  increased 
vigor  of  reproduction.  We  shall  discuss  this  matter  farther 
in  dealing  with  the  question  of  the  production  of  inherited 
variation  by  conjugation.  But  if  we  base  our  conclusions  on 
the  usual  or  the  average  effect  of  conjugation,  we  are  forced 
to  conclude  that  in  most  cases  it  does  not  increase  the  rate 
of  multiplication,  but  rather  it  decreases  this  rate. 

It  is  not  generally  realized  that  the  work  of  Maupas 
and  of  Richard  Hertwig,  on  whose  authority  the  theories  of 
rejuvenescence  by  conjugation  largely  rest,  was  squarely 
opposed  to  the  idea  that  this  rejuvenescence  manifests  itself 
by  an  increase  of  reproductive  vigor.  It  is  worth  while  to 
2  Calkins,  G.  N.,  Proc.  Nat.  Acad.,  April,  1919. 


Conjugation  and  Rate  of  Reproduction  145 

emphasize  this  point,  for  upon  it  an  extraordinary  error  has 
become  widespread  even  among  expert  biologists.3 

Maupas  rejected  emphatically  the  idea  that  aging  shows 
itself  in  a  decrease  of  the  rate  of  reproduction  and  that 
conjugation  restores  this  declining  rate.  He  asserted  posi- 
tively that  no  such  decline  occurred  before  conjugation  and 
that  after  conjugation  reproduction  was  not  more  rapid 
than  before,  and  he  based  these  statements  on  the  results  of 
careful  experimentation.  Richard  Hertwig's  experiments 
led  to  the  same  result  and  induced  him  to  subscribe  com- 
pletely to  Maupas'  assertions  on  this  point.  The  matter  is 
important,  since  the  doctrine  that  conjugation  restores  the 
declining  fission  rate  depends  largely  on  their  supposed  sup- 
port of  it.  I  therefore  give  in  their  own  words  statements 
of  the  results  of  their  experiments,  and  of  the  conclusions 
they  drew  from  these  results. 

Maupas  sums  up  the  prevailing  mistaken  doctrine  of  his 
time  on  this  matter  in  words  which  well  fit  the  present  con- 
ditions : 

"It  has  been  affirmed  that  the  faculty  of  reproducing  by 
fission  in  the  ciliates  was  modified  by  conjugation,  and  that 
the  principal  effect  of  this  sexual  act  was  to  reenforce  and 
accelerate  this  reproduction.  The  ciliates,  it  was  held,  im- 
mediately after  conjugation  multiply  more  rapidly  than 
they  do  at  a  later  stage.  This  opinion  has  become  current, 
and  one  finds  it  reproduced  in  Memoirs  and  in  general  trea- 
tises, as  if  it  were  a  definitely  established  truth"  (1888,  p. 
254-255). 

Maupas  then  proceeds  to  show  that  the  evidence  on  which 

•Thus,  investigators  so  experienced  as  Woodruff  and  Erdmann  in  a 
recent  paper  (1914)  contrast  my  own  results  on  this  point  with  "the 
view  of  Butschli,  Maupas,  Hertwig  and  others,"  asserting  that  accord- 
ing to  the  latter,  after  mating  "all  the  processes  of  the  cell,  including 
reproduction,  proceed  with  greater  vigor." 


146         Life  and  Death,  Heredity  and  Evolution 

this  theory  had  been  based  amounted  to  nothing.  Next  he 
sets  forth  that  in  his  own  records  of  fissions  in  different  in- 
fusoria, beginning  in  a  number  of  cases  with  animals  that 
had  just  conjugated,  there  is  no  indication  of  a  greater 
rate  of  fission  after  conjugation.  He  says  of  the  fis- 
sions : 

"They  succeed  one  another  uniformly,  modified  only  by 
the  changes  of  temperature.  I  did  not  remain  content  with 
this  one  experiment.  I  isolated  other  ex-con jugants  of 
Stylonychla  pustulata,  of  Onychodromus  grandis,  of 
Euplotes  patella,  of  Paramecmm  aurelia,  of  Leucophrys 
patula.  I  followed  day  by  day  the  successive  generations  of 
their  descendants,  during  periods  of  from  fifteen  days  to  two 
months.  In  none  of  these  species  did  I  see  any  difference  in 
the  succession  of  fissions.  Whether  they  had  conjugated 
lately  or  a  long  time  ago,  all  the  individuals  acted  in  the 
same  way"  (1888,  p.  255-256). 

Maupas  sums  up  the  matter  in  a  later  paper  as  follows: 

"I  have  asserted,  besides,  that  this  power  of  multiplica- 
tion is  maintained  regularly  and  uniformly  during  the  entire 
life  cycle;  that  there  is  no  gradual  weakening  of  this  power 
from  the  first  generation  after  conjugation  up  to  the  re- 
turn of  a  new  period  of  maturity.  In  other  words,  I  deny 
that  the  infusoria  after  conjugation  have  a  more  energetic 
reproduction  than  they  have  at  a  later  period"  (1889,  p. 
504). 

Certainly  this  is  sufficiently  explicit  not  to  be  misunder- 
stood !  It  is  because  Maupas'  papers,  with  their  hundreds  of 
pages  of  text  filled  with  observational  and  experimental  de- 
tails, make  hard  reading,  that  it  is  possible  for  mistaken 
ideas  of  his  results  to  become  prevalent. 

Richard  Hertwig  (1889)  found  in  his  experiment,  to  his 
surprise  (as  will  anyone  that  tries  it),  not  only  that  the 


Theories  of  Rejuvenescence  147 

Paramecia  which  had  conjugated  did  not  reproduce  faster, 
but  that,  on  the  contrary,  it  was  those  that  had  not  been 
allowed  to  conjugate  that  multiplied  more  rapidly.  Speak- 
ing of  the  theory  that  conjugation  increases  the  energy 
of  reproduction,  Hertwig  says : 

"The  grounds  on  which  this  theory  is  based  have  already 
been  combated  by  Maupas ;  he  showed  by  extended  experi- 
ments that  the  power  of  multiplication  of  an  infusorian  is 
neither  decreased  before  conjugation  nor  increased  after  it. 
...  I  am  compelled  to  say  that  Maupas  was  entirely  right" 
(1889,  p.  222). 

What  then  did  Maupas  and  Hertwig  mean  by  holding  that 
conjugation  does  nevertheless  rejuvenate? 

Maupas'  theory  is  not  easy  to  seize  or  to  state  in  experi- 
mental terms.  He  believed  that  without  conjugation  the 
organisms  became  deformed  and  structurally  degenerate, 
although  their  power  of  reproduction  remained  unimpaired ; 
at  last  they  died.  Conjugation,  he  believed,  if  it  occurs 
before  degeneration  has  become  evident,  prevents  the  process 
of  degeneration.  His  own  experiments  showed  him,  as  we 
shall  see,  that  after  observable  degeneration  has  begun  con- 
jugation does  not  remedy  it.  The  only  test  for  Maupas' 
theory  is  formed  by  such  experiments  as  those  of  Woodruff, 
in  which  it  is  shown  that  even  without  conjugation  the  ani- 
mals live  indefinitely  and  do  not  degenerate. 

Hertwig  (1889),  taking  the  bull  by  the  horns,  held  with 
relation  to  Paramecium,  that  an  increase  in  the  rate  of  re- 
production was  one  of  the  symptoms  of  degeneration;  that 
conjugation  in  restoring  the  balance  decreased  the  rapidity 
of  fission.  It  is  extraordinary  that  two  such  contradictory 
views  can  be  held  of  what  conjugation  does,  as  are  these  two 
— Hertwig  on  the  one  hand  maintaining  that  conjugation 
decreased  the  rate  of  reproduction,  Calkins  on  the  other 
that  it  increases  it — both  under  the  name  of  rejuvenescence! 


148          Life  and  Death,  Heredity  and  Evolution 

Hertwig  has  developed  later  a  general  theory  of  mating, 
rejuvenescence  and  kindred  matters;  a  theory  which  is  at 
once  extremely  special  and  extremely  indefinite.  It  is  based 
upon  ideas  of  a  necessary  proportion  in  quantity  between  the 
mass  of  the  nucleus  and  that  of  the  cytoplasm.  As  growth 
occurs  the  cytoplasm  increases  faster  than  the  nucleus,  so 
that  after  a  time  the  nucleus  is  too  small  in  proportion  to  the 
amount  of  cytoplasm.  This  brings  about  in  some  way  a  sud- 
den increase  in  the  growth  of  the  nucleus,  and  this  in  turn 
causes  division  of  the  cell.  In  the  course  of  many  cell 
generations,  through  irregularities  in  division,  and  through 
the  action  of  various  agents,  the  nucleus  may  become  too 
large  in  proportion  to  the  cytoplasm.  Enlargement  of  the 
nucleus  has  been  found  to  occur  in  cold;  in  hunger;  when 
the  animals  are  overfed;  and  in  various  other  conditions. 
In  such  conditions  the  animals  do  not  divide  frequently, 
and  this  is  attributed  by  Hertwig's  theory  to  the  dispropor- 
tionate size  of  the  nuclei.  Such  conditions  of  large  nuclei 
and  slow  reproduction,  with  other  pathological  symptoms, 
are  spoken  of  as  states  of  "depression."  Such  depression 
can  be  remedied,  if  it  has  not  gone  too  far,  in  various  ways, 
— by  change  of  temperature,  of  food,  and  the  like ;  the  essen- 
tial thing  that  then  happens,  according  to  this  view,  is  that 
the  nucleus  reduces  its  size,  by  throwing  out  substance  into 
the  cytoplasm,  or  otherwise.  But  if  depression  has  gone 
too  far  for  this,  it  can  be  overcome  only  by  a  deep-seated 
reorganization  of  the  nucleus.  Such  a  deep-seated  reor- 
ganization occurs  in  the  processes  connected  with  mating; 
hence,  it  is  held,  these  processes  restore  the  normal  balance 
of  nucleus  and  cytoplasm ;  the  depression  disappears,  and 
the  organisms  continue  to  live  and  multiply. 

Such  a  theory  appears  artificial  and  based  upon  super- 
ficial features.  When  one  considers  the  complex  chemical 


Theories  of  Rejuvenescence  149 

processes  that  lie  at  the  foundation  of  life,  it  is  difficult 
to  believe  that  mere  relations  of  proportionate  quantity  of 
two  things  so  complex  as  nucleus  and  cytoplasm,  both  having 
the  power  of  assimilation  and  growth,  is  at  the  bottom  of 
these  matters  of  life,  death  and  reproduction.  As  Dobell 
(1909)  expresses  it,  one  would  almost  "as  soon  argue  that 
grey  hairs  are  the  cause  of  old  age  in  man"  as  to  hold 
that  increase  in  size  of  the  nuclei  is  the  cause  of  degeneration 
and  death  in  cells.  Furthermore,  this  theory  gives  absolutely 
no  ground  for  the  chief  feature  of  mating, — the  fact  that  it 
is  a  mating  of  two  nuclei,  not  a  mere  reorganization.  And 
like  the  other  theories  of  rejuvenescence,  its  validity  depends 
finally  on  whether  mating  actually  does  restore  vigor,  vital- 
ity and  reproductive  power  to  depressed  organisms.  Most 
of  such  theories  have  proceeded  on  the  ground  that  if  one 
can  show  that  degeneration  occurs  without  conjugation,  then 
it  must  follow  that  mating  remedies  the  degeneration.  The 
real  test  lies  in  observing  whether  mating  actually  does  rem- 
edy the  degeneration.  If  it  does  not,  the  probability  be- 
comes strong  that  the  degeneration  is  simply  a  pathological 
result  of  bad  conditions.  The  evidence  is  becoming  over- 
whelming that  this  is  the  case ;  that  to  avoid  degeneration, 
it  is  merely  necessary  to  avoid  the  bad  conditions.  The  work 
of  Woodruff,  set  forth  in  our  first  lecture,  goes  far  to  dem- 
onstrate this. 

Hertwig's  own  test  of  the  matter  in  Paramecium  was,  as 
we  have  seen,  opposed  to  the  idea  that  mating  increases 
the  vigor  of  reproduction ;  Maupas'  extensive  work  gave  the 
same  result,  and  my  own  still  more  extensive  experiments 
led  to  the  same  conclusion.  Only  Calkins'  experiments  with 
Uroleptus  stand  in  the  way  of  asserting  this  to  be  the  general 
rule  in  infusoria. 

Certainly  no  clear  case  has  been  made  out  for  rejuvenes- 


150          Life  and  Death,  Heredity  and  Evolution 

cence  as  a  general  result  of  mating.  Indeed  it  is  only  fair  to 
say  that  most  of  the  positive  evidence  is  against  rejuvenes- 
cence of  any  sort.  In  Paramecium,  not  only  do  those  that 
have  conjugated  multiply  less  rapidly  than  before,  but  a 
great  many  of  them  die  after  conjugation, — although  with- 
out conjugation  they  live  vigorously.  Many  others  are  weak 
and  sickly,  multiplying  but  little.  Still  further,  conjugation 
produces  a  great  number  of  abnormalities  and  monstrosities, 
such  as  do  not  occur  without  mating  (see  Figure  44).  In 
many  other  infusoria  all  or  nearly  all  those  that  have  con- 
jugated die,  or  completely  cease  multiplying.  Maupas  made 
many  investigations  giving  such  results.  He  allowed  two 
unrelated  cultures  of  Stylonychia  to  become  degenerated, 
then  let  them  conjugate  together;  according  to  the  theory 
this  ought  to  have  rejuvenated  them.  But  it  did  not;  they 
all  died.  He  found  that  even  in  wild  Stylonychias  conjuga- 
tion rather  usually  results  in  death ;  later  students  have  con- 
firmed this.  In  his  experiments,  Maupas  found  that  the 
individuals  that  have  conjugated  all  die  in  Spirostomum, 
Climacostomum  and  Didinium.  In  Leucophrys  he  found 
that  a  large  proportion  of  those  that  mate  die.  It  used  to 
be  supposed  that  such  results  were  due  to  the  fact  that  the 
animals  had  become  so  degenerate  that  even  conjugation 
could  no  longer  save  them.  But  we  now  know  that  under 
proper  conditions,  without  conjugation  they  would  (in  most 
cases  at  least)  live  and  multiply  vigorously,  while  after  con- 
jugation they  are  weak  or  die. 

Such  facts  are  most  extraordinary;  they  are  difficult  to 
explain  on  any  theory.  A  priori  it  appears  that  the  re- 
placement of  the  old  active  nucleus  by  the  reserve  nucleus, 
must  tend  to  give  rejuvenescence  in  conjugation,  just  as 
when  it  occurs  without  conjugation.  But  this  tendency  is  in 
the  majority  of  cases  completely  overwhelmed  and  done  away 
with  by  other  features  of  mating;  by  the  physiological  diffi- 


Distinctive  Results  of  Mating  151 

culties  of  the  complex  process ;  and  as  we  shall  try  to  show, 
by  frequent  incompatibilities  between  the  parts  united. 

In  rare  lines  in  Paramecium,  and  other  infusoria,  and 
commonly  in  Uroleptus,  according  to  Calkins,  all  these  diffi- 
culties are  overcome,  so  that  the  replacement  of  the  worn 
nucleus  by  the  reserve  has  the  same  effect  in  renewing  vitality 
that  it  has  when  its  occurs  without  mating.  There  is  thus 
far  no  evidence  either  from  Calkins'  experiment  with  Urolep- 
tus, or  from  any  others,  that  the  mating,  as  distinguished 
from  the  replacement  of  tLc  macronucleus  by  the  micronuc- 
leus,  produces  rejuvenescence.  Calkins  finds  that  in  Urolep- 
tus this  replacement  without  conjugation  restores  vitality 
as  it  does  with  conjugation. 

What  then  are  the  distinctive  results  of  mating,  as  com- 
pared with  that  replacement  which  occurs  without  mating? 
On  this  the  theory  of  rejuvenescence  has  not  thus  far  cast 
light.  The  thing  to  do  under  such  circumstances  is  to  drop 
for  the  time  any  definite  preconceived  theory,  and  examine 
with  care  the  facts ;  sometimes  these  carry  a  theory  of  their 
own!  Just  what  differences  can  we  find  between  individuals 
that  have  mated  and  those  that  have  not? 

In  our  examination  of  the  question  of  diversity  of  sex  in 
Paramecium  (Lecture  5),  we  mentioned  a  difference  that  was 
found  in  that  organism.  After  mating,  the  two  individuals 
and  their  offspring  have  become  more  alike  than  the  two 
stocks  were  before  mating.  This,  as  before  remarked,  is  not 
unnatural,  for  each  individual  after  mating  is,  if  it  may 
be  so  expressed,  partly  made  up  from  the  other  individual. 
In  all  of  the  offspring  of  each  of  the  mates,  half  the  nucleus 
has  come  from  the  other  mate.  It  is  therefore  not  surprising 
that  the  two  sets  of  offspring  are  much  alike;  they  are  all 
children  of  the  same  family ;  they  are  as  closely  related  as 
are  the  brothers  and  sisters  of  a  human  family. 

Here  then  we  have  in  these  lower  organisms  a  distinctive 


152          Life  and  Death,  Heredity  and  Evolution 

visible  difference  made  by  mating,  and  it  is  the  same  sort  of 
result  that  is  produced  by  mating  in  higher  organisms.  In 
the  latter  in  consequence  of  mating,  offspring  are  produced 
which  are  more  alike  than  are  individuals  taken  at  random; 
this  is  likewise  what  happens  in  Paramecium. 

In  higher  organisms  we  say  that  this  result  is  an  example 
of  heredity.  As  we  are  accustomed  to  think  of  it,  the  off- 
spring in  a  family  inherit  from  both  their  parents ;  they 
resemble  both  the  parents ;  and  this  appeals  to  us  as  a  reason 
why  they  should  resemble  each  other.  Now,  whether  the  off- 
spring of  the  two  members  of  a  pair  of  Paramecia  resemble 
also  their  parents  has  not  been  directly  shown;  it  is  a  dif- 
ficult matter  to  get  at;  what  we  know  is  that  they  do 
show  a  marked  resemblance  to  each  other.  But  this  matter 
of  resemblance  between  the  external  features  of  parent  and 
offspring  is  in  reality  a  totally  inadequate  basis  for  a  con- 
ception of  heredity.  In  the  study  of  heredity  in  higher 
organisms,  we  often  discover  that  the  offspring  inherit  char- 
acteristics from  the  parents  which  the  parents  do  not  show, 
— as  when  rabbits  with  white  hair  are  born  from  parents 
with  brown  hair;  or  when  two  white  fowls  produce  black 
chicks.  In  such  cases  the  black  color  of  the  chicks  is  in- 
herited from  the  white  parents  just  as  truly  and  in  just  the 
same  way  as  occurs  when  black  parents  produce  black 
chicks.  The  only  consistent  meaning  we  can  give  to  heredity 
is  this :  heredity  is  the  appearance  of  peculiarities  in  the  off- 
spring that  are  due  to"  the  peculiarities  of  the  germinal 
material  they  obtained  from  their  parents.  And  we  know 
that  white  fowls  do  frequently  transmit  to  their  offspring 
germ  cells  of  such  a  character  that  the  offspring  must  be 
black;  such  cases  are  recognized  as  clear  examples  of  Men- 
delian  inheritance.  By  biparental  inheritance  we  mean  mere- 
ly this:  the  peculiarities  of  the  given  individual,  taken  as  a 


Biparental  Inheritance  and  Conjugation  153 

whole,  are  due  to  its  origin  from  the  united  germinal  ma- 
terial from  two  parents;  if  the  individual  had  been  derived 
from  but  one  parent,  it  would  have  shown  other  peculiarities. 
Thus  when  two  kinds  of  white  flowers  produce  by  their  union 
plants  with  red  flowers,  this  is  an  example  of  biparental  in- 
heritance (and  one  of  not  uncommon  kind), — for  the  red 
color  would  not  have  occurred  if  the  two  parents  had  not 
united,  as  for  example  in  reproduction  through  cuttings. 

Now,  the  resemblance  between  the  offspring  of  the  two 
members  of  a  pair  of  infusoria  is  due  to  the  fact  that  they 
have  received  germinal  material  (the  two  halves  of  their 
micronuclei)  from  both  parents.  It  is  therefore  an  example 
of  biparental  inheritance. 

I  have  given  this  brief  exposition  of  what  heredity  means, 
and  particularly  of  what  biparental  inheritance  means,  be- 
cause to  persons  not  familiar  with  experimental  work  in 
heredity,  it  appears  surprising  that  we  should  speak  of  bi- 
parental inheritance  when  we  haven't  proved  that  the  off- 
spring resemble  their  two  parents;  this  objection  has  been 
made  by  Dobell  (1914).  But  as  a  matter  of  fact  we  speak 
of  inheritance  equally  when  we  know  that  the  offspring  do 
not  resemble  their  parents, — provided  their  peculiarities  are 
due  to  the  germinal  material  derived  from  their  parents. 
The  other  significance  for  heredity  would  exclude  a  large 
proportion  of  the  best  known  cases  of  Mendelian  inherit- 
ance. 

-  We  find  then  that  in  these  Protozoa,  as  in  higher  organ- 
isms, mating  results  in  inheritance  from  both  parents.  In 
what  respects  does  such  inheritance  occur?  Our  knowledge 
on  this  point  is  scanty ;  until  recently  it  was  confined  to  cer- 
tain characteristics  of  Paramecium;  to  this  has  now  been 
added  important  knowledge  of  these  matters  in  Chlamydo- 
monas.  This  is  one  of  the  lines  of  work  in  which  there  is 


154          Life  and  Death,  Heredity  and  Evolution 

most  room  for  acquiring  valuable  knowledge;  but  it  is  ex- 
tremely difficult  to  carry  out  the  necessary  experiments. 
The  facts  for  Paramecium  are  as  follows : 

•  I]  In  Paramecium,  biparental  inheritance  occurs  with 
reference  to  size;  after  conjugation  the  two  sets  of  progeny 
are  more  alike  in  size  than  before  (See  Jennings  and  Lash- 
ley,  1913a). 

(£)  It  occurs  also  with  relation  to  rate  of  fission.  Here 
it  is  often  very  striking.  If  the  descendants  of  one  member 
of  a  given  pair  multiply  rapidly,  the  descendants  of  the  other 
member  are  likely  to  multiply  rapidly  also ;  if  one  set  multi- 
ply slowly,  so  also  as  a  rule  does  the  other  set.  One  or  two 
examples  (from  Jennings  and  Lashley,  1913)  of  this  are 
perhaps  worth  while.  Calling  the  two  members  of  a  given 
pair  a  and  b,  the  numbers  of  fissions  for  four  successive 
periods  of  ten  days  each  were  for  several  pairs,  all  under  the 
same  conditions : 


Pair  Total 

.,     fa  10  11  10  12  =  43 

1    \b  10  10  12  15  =  47 

,7     fa  5  5  5  4  =  19 

'     \b  5  7  4          5  =  21 

Q    /a  9  7  8          8  =  32 

Mb  8  6  6          9  =  29 


ina     fa        11         11         13         13  =  48 
108    |b        n 


13        12         13  =  49 


As  you  see,  both  members  of  pair  41  divide  rapidly;  of 
pair  47  slowly ;  of  9,  at  an  intermediate  rate ;  while  in  pair 
108  both  members  divide  still  more  rapidly  than  in  pair 
41.  This  condition  of  affairs  is  typical. 

(3)  There  is  a  similar  resemblance  between  the  offspring 
of  the  two  members  in  respect  to  vigor  and  vitality.  Some- 
times the  descendants  of  one  pair  are  weak  and  the  stock 
gradually  dies  out.  When  this  happens,  the  family  derived 


Biparental  Inheritance  and  Conjugation  155 

from  the  other  member  is  likely  to  be  weak  and  die  out  also. 
This  was  studied  with  very  great  thoroughness,  and  a  strong 
tendency  to  likeness  in  this  respect  was  found  (Jennings  and 
Lashley,  1913). 

(4)  A  fourth  respect  in  which  the  offspring  of  the  two 
that  have  mated  tend  to  be  alike  is  in  certain  structural  ab- 
normalities that  often  occur  (Stocking,  1915).  Such  abnor- 
malities are  shown  in  Figure  44.  Frequently  these  abnor- 
malities are  hereditary  in  the  family  derived  from  one  mem- 
ber of  a  pair ;  when  this  is  the  case,  the  family  derived  from 
the  other  member  often  shows  such  abnormalities  also, — 
much  more  frequently  than  is  the  case  when  its  mate  is  nor- 
mal. 

In  all  respects  in  which  the  matter  has  been  examined, 
therefore,  mating  tends  to  make  -the  families  derived  from 
the  two  members  of  a  pair  more  alike  than  they  would  have 
been  without  it.  Conjugation  produces  biparental  inherit- 
ance, just  as  fertilization  does  in  higher  organisms. 

In  Chlamydomonas,  Pascher 4  has  recently  succeeded  in 
studying  the  results  of  conjugation  with  respect  to  charac- 
ters of  a  more  tangible  sort  than  those  that  we  were  forced  to 
use  in  Paramecium.  Two  species  of  Chlamydomonas  differed 
in  form  and  structure  in  the  way  shown  in  Figure  43a,  A  and 
B.  In  conjugation  these  organisms  unite  completely,  form- 
ing a  cyst.  The  cyst  formed  by  the  conjugation  of  two  in- 
dividuals like  A  has  no  membrane  and  is  covered  with  pyra- 
midal elevations,  as  shown  at  AA,  while  that  formed  by  two 
like  B  is  smooth  and  surrounded  by  thick  membranes  (BB). 
When  A  conjugates  with  B,  the  resulting  cyst  is  intermediate 
between  the  two  pure  forms,  as  shown  at  AB ;  there  is  a  thin 
layer  of  membranes,  and  the  surface  is  covered  with  low 
rounded  elevations. 

4  Pascher,  A.,  Ber.  d.  Deutsch.  Bot.  Qes.,  1916  and  1918. 


156          Life  and  Death,  Heredity  and  Evolution 


Figure  43a.  Biparental  inheritance  and  the  production  of  diversity 
by  conjugation,  in  Chlamydomonas,  after  Pascher  (1916),  from  Hart- 
mann.1  A  and  B,  the  two  parental  species.  AA,  cyst  formed  by  con- 
jugation of  two  individuals  such  as  A.  BB,  cyst  formed  by  conjugation 
of  B.  AB,  cyst  formed  by  conjugation  of  A  with  B.  a,  b,  c,  d,  the 
four  types  of  free  living  individuals  resulting  from  the  conjugation  of 
A  and  B. 

In  this  cyst  the  reducing  divisions  occur  (Lecture  VII), 
and  it  divides  into  four  free-swimming  individuals  which 
when  the  conjugation  was  between  two  similar  parents,  are 
like  those  parents.  These  free  individuals  now  reproduce  by 
fission  in  the  usual  way. 

When  the  cyst  had  been  formed  by  conjugation  of  the  two 
diverse  kinds  of  parents  A  and  B,  the  progeny  show  a  num- 
'Hartmann,  M.,  Zeitschr,  f.  Ind.  Abst.f  September,  1918, 


Diversity  Produced  by  Conjugation  157 

her  of  diverse  combinations  of  the  characters  of  the  two  par- 
ents, as  shown  in  Figure  43a,  a,  b,  c,  d.  Pascher  (1918) 
gives  a  table  of  the  various  combinations  found  in  the  four 
sets  of  individuals  in  comparison  with  the  characteristics  of 
the  parents  A  and  B,  as  follows : 

Form        Membrane    Papilla  Chromatophore  Eye  spot 
Parent  A        pear-shaped        delicate        none        lateral        linear 
Parent  B  spherical  coarse     present        basal          broad 


Offspring  a  pear-shaped 
b  pear-shaped 
"          c    ellipsoidal 
"          d     spherical 

delicate 
delicate 
coarse 
coarse 

none 
none 
present 
present 

lateral 
basal 
lateral 
basal 

linear 
broad 
linear 
broad 

The  offspring  a  and  d  resemble  very  closely  the  two  par- 
ents respectively,  while  the  types  b  and  c  show  characters  of 
both  the  parental  forms.  This  is  the  same  sort  of  result 
which  we  find  in  the  Mendelian  inheritance  of  higher  organ- 
isms. It  is  a  demonstrated  fact,  therefore,  that  in  the 
Protozoa,  as  in  higher  forms,  conjugation  results  in  bi- 
parental  inheritance. 

Observe  also  that  besides  producing  resemblance  of  par- 
ents to  progeny  or  of  progeny  to  one  another,  conjugation 
causes  stocks  or  races  or  families  to  arise  which  are  diverse 
in  their  hereditary  characters  from  either  of  those  from 
which  they  took  origin.  This  is  evident  in  Chlamydomonas ; 
b  and  c  show  diverse  combinations  from  those  found  in 
either  of  the  parents,  and  these  diversities  are  perpetuated 
in  their  propagation  by  fission.  In  Paramecium,  from  a 
single  race — all  having  the  same  hereditary  characters — 
there  may  arise  by  conjugation  many  races  with  diverse 
hereditary  characters. 

What  this  means  will  best  be  illustrated  from  an  actual 
experiment  (from  Jennings,  1913).  We  begin  with  a  set  of 
individuals  all  alike,  all  derived  by  fission  from  a  single 
parent.  We  study  their  rate  of  fission ;  taking  174  separate 


158          Life  and  Death,  Heredity  and  Evolution 

lines  of  descent,  we  find  it  to  be  extremely  uniform.  For 
periods  of  21  days  we  find  the  number  of  fissions  of  the 
first  24  lines  to  be  those  given  under  "Non-con jugants," 
a  and  b,  in  the  following  table.  (The  two  individuals,  a 
and  b,  of  the  non-con  jugants,  had  begun  to  mate,  but  were 
separated  before  the  mating  process  occurred.)  In  these  24 
lines  the  number  of  fissions  in  21  days  varied  only  from  21 
to  26 ;  it  is  on  the  whole  very  uniform.  One  finds  little  or 
no  indication  of  inherited  differences  between  the  families; 
they  run  very  evenly. 

Now  we  allow  a  large  number  of  the  individuals  to  con- 
jugate, then  follow  the  rate  of  fission  for  88  lines  derived 
from  these  mates.  We  find  that  there  are  now  great  dif- 
ferences between  the  separate  families  with  respect  to  fission 
rate.  In  the  table  the  two  last  columns,  headed  "Con- 
jugants," give  the  number  of  fissions  for  24  lines  descended 
from  pairs  of  conjugants,  for  the  same  21  days  as  for 
those  that  have  not  conjugated.  The  two  sets  (Non- 
con  jugants  and  Conjugants)  were  kept  throughout  under 
the  same  conditions. 

Table 

Comparative  fission  numbers  in  21  days  of  Conjugants  and  Non- 
con  jugants  derived  from  a  single  race  E  (from  Tables  34  and  35; 
Jennings,  1913).  In  the  non-con  jugants,  a  and  b  represent  two  indi- 
viduals that  had  begun  to  mate,  but  were  separated  before  mating  was 
accomplished.  In  the  conjugants,  a  and  b  are  the  two  mates  from  one 
pair. 

Non-con  jugants  Conjugants 

Pair       a          b  a          b 

1  25        23  30        29 

2  24        23  25        24 

3  24        24.  29        28 

4  24        25  10          9 

5  23        24  8        10 

6  22        25  24        24 

7  22        26  29        28 

8  31        25  25        24 

9  24        25  30        27 

10  22        26  26        26 

11  23        24  28        27 

12  25        24,  11          9 


Diversity  Produced  by  Conjugation  159 

In  the  descendants  of  those  that  have  conjugated  the 
number  of  fissions  in  21  days  varies  from  8  to  30,  as 
compared  with  21  to  26  in  the  others.  There  are  now 
strongly  marked  differences  between  the  different  families 
that  have  descended  from  those  that  have  conjugated.  It  is 
interesting  to  compare  the  records  of  fissions  by  two-day 
periods  for  a  considerable  time  in  such  families  derived 
from  different  ex-con jugants,  Typical  examples  are  as 
follows : 

Family 

lax  02223444333      3  =  33 

6  ax  021111122212  =  16 

9  ax  122344333334  =  35 

27  ax  13112222223       1=22 

15  ax  222212010100  =  13 

All  these  families  were  kept  under  the  same  conditions, 
yet  they  consistently  differ  in  their  fission  rates  throughout 
the  entire  period.  They  have  become  hereditarily  diverse 
in  this  respect,  as  a  result  of  conjugation,  for  all  were 
derived,  before  conjugation,  from  the  same  original  par- 
ent, and  all  had  the  same  rates  of  fission.  Conjugation 
has  produced  from  a  single  stock  a  number  of  hereditarily 
diverse  stocks.  The  same  thing  was  demonstrated  in  a 
number  of  extensive  experiments. 

Another  respect  in  which  conjugation  increases  the 
hereditary  differentiation  is  in  the  matter  of  abnormalities. 
If  we  begin  with  a  set  of  the  animals  that  are  all  quite 
normal,  showing  no  inherited  abnormalities,  and  allow  them 
to  conjugate,  we  often  find  that  many  of  the  families  pro- 
duced have  hereditary  abnormalities,  while  others  have  none. 
Such  hereditary  abnormalities,  produced  as  a  result  of  con- 
jugation, are  shown  in  Figure  44.  In  work  by  Dr.  Stock- 
ing (1915),  of  450  families  descended  from  animals  that 
have  conjugated,  262,  or  more  than  half,  showed  hereditary 


160          Life  and  Death,  Heredity  and  Evolution 


1 


(3. 


ABNORMAL 
ABNORMAL 


ABNORM 


//  /  I  ABNORMAL  ABNORMAL  ABNORMAL  ABNORMAL 


DEAD 


ABHOR  MAI,. 


DEAD 


Figure  44.  A  family  of  Paramecium  caudatum,  descended  from  an 
ex-con j  ugant,  and  showing  hereditary  abnormalities.  Besides  the  indi- 
viduals figured,  other  abnormal  branches  of  the  family  are  indicated 
by  the  lines  with  the  word  "abnormal."  After  Stocking,  1915. 


abnormalities, — although  none  were  present  before  conjuga- 
tion. 


Diversity  Produced  by  Conjugation  161 

There  is  evidence  also  that  conjugation  causes  hered- 
itary differentations  to  appear  with  respect  to  size,  but 
this  matter  has  not  been  so  precisely  studied  as  it  deserves 
to  be. 

Calkins  and  Gregory  (1913)  observed  similar  hereditary 
variations  in  fission  rate  and  vitality  arising  after  con- 
jugating. They  give  some  evidence  that  hereditarily  differ- 
ent families  may  arise  from  the  first  two  divisions  of  one 
of  the  members  of  a  pair.  During  conjugation,  as  we  have 
seen,  new  macronuclei  are  produced  from  the  reserve,  micro- 
nucleus  (see  Figure  30).  The  first  four  of  these  are  pro- 
duced within  the  individual  that  has  conjugated,  and  be- 
fore reproduction  takes  place.  Then  by  two  divisions  these 
are  distributed  to  the  four  progeny.  According  to  the  re- 
sults of  Calkins  and  Gregory,  the  four  individuals  receiving 
the  four  macronuclei  thus  produced  may  become  diverse; 
probably,  it  would  seem,  through  the  diversity  of  these  four 
nuclei.  In  later  divisions  such  diversities  do  not  appear, 
according  to  their  account ;  at  least  they  say  that  each  of 
the  lines  produced  remains  "true  to  its  type  for  many 
months  at  least."  I  believe  that  this  matter  requires 
further  study,  but  according  to  the  results  of  Calkins  and 
Gregory,  these  inherited  differences  are  strictly  the  result  of 
conjugation,  just  as  are  those  shown  in  my  own  work. 

We  find  therefore  that  in  all  the  hereditary  characters  in 
which  the  matter  has  been  studied,  conjugation  gives  rise  to 
inherited  differences ;  in  other  words,  diverse  stocks  arise 
as  a  result  of  conjugation.  Such  work  needs  to  be  greatly 
extended;  all  we  know  on  the  matter  is  based  on  Chlamydo- 
monas  and  Paramecium,  mainly  the  latter,  and  its  characters 
are  not  so  favorable  for  such  work  as  might  perhaps  be 
found.  But  such  work  is  extremely  difficult.5 

"Certain  work  of  Mast  (1917),  bearing  on  this  point,  in  the  infu- 
sorian  Didinium,  will  be  taken  up  in  our  next  lecture. 


162          Life  and  Death,  Heredity  and  Evolution 

We  must  now  revert  for  a  moment  to  the  relation  of  this 
production  of  diverse  stocks  through  mating  to  the  problem 
of  rejuvenescence.  Among  the  characters  in  which  mat- 
ing induces  hereditary  diversities  is  the  rate  of  fission. 
This  has  been  illustrated  on  page  158. 

Now,  as  we  have  seen,  if  we  take  all  the  lines  that  have 
conjugated  and  average  them,  we  find  that  their  average 
fission  rate  is  somewhat  less  than  the  average  rate  of  those 
that  have  not  conjugated.  Thus  in  a  very  extensive  ex- 
periment in  which  69  lines  derived  from  conjugants  were 
compared  for  a  period  of  three  weeks  with  145  lines  derived 
from  non-con jugants,  the  daily  fission  rate  of  the  con- 
jugants averaged  1.097,  that  of  the  non-con  jugants  1.1 44 
(Jennings,  1913,  p.  349).  For  the  period  of  21  days  the 
average  number  of  fissions  in  each  line  was  for  the 
descendants  of  the  conjugants  23.041 ;  for  the  descendants 
of  the  non-con  jugants,  24.034.  When  we  study  the  separate 
lines  we  find  that  those  descended  from  the  non-con  jugants 
all  show  nearly  the  same  number  of  fissions ;  the  slowest  line 
had  18  fissions,  the  fastest  28,  in  21  days.  But  the  lines 
descended  from  the  conjugants  differ  greatly  among  them- 
selves; the  slowest  line  has  for  the  twenty-one  days  only 
9  fissions,  while  the  most  rapid  one  has  31.  That  is, 
mating  has  caused  much  hereditary  diversity  in  the  fission 
rate  of  the  descendants  of  the  conjugants,  and  those  at  one 
extreme  of  the  variation  have  a  higher  fission  rate  than 
those  descended  from  the  non-conjugants.  Out  of  the  de- 
scendants of  56  ex-con  jugants,  in  this  experiment,  9  pro- 
duced families  with  a  higher  fission  rate  than  any  of  the 
families  descended  from  the  130  non-conjugants.  On  the 
other  hand,  9  of  the  families  produced  from  the  ex- 
con  jugants  had  a  lower  fission  rate  than  the  low  family  (with 
18  fissions)  produced  from  the  non-conjugants. 


Diversity  Produced  by  Conjugation  163 

The  descendants  of  the  ex-con jugants  do  not  by  any 
means  always  show  some  families  that  exceed  the  fission 
rate  in  the  descendants  of  the  non-conjugants.  In  nine  ex- 
tensive cultures  (described  in  my  paper  of  1913,  page  364) 
in  which  this  matter  was  studied,  in  but  three  did  the 
maximum  for  the  con  jugants  exceed  that  for  the  non-con- 
jugants, while  in  all  cases  the  minimum  and  the  average  for 
the  conjugants  were  below  those  for  the  non-conjugants. 

We  have  set  forth  the  relations  found  in  Paramecium,  the 
organism  most  thoroughly  studied  from  this  point  of  view. 
In  Uroleptus,  according  to  Calkins,  those  that  have  conju- 
gated usually  show  an  increased  rate  of  reproduction.  The 
production  among  the  combinations  resulting  from  mating, 
of  families  with  an  increased  vigor  of  reproduction  (usual 
in  some  infusoria,  occasional  or  rare  in  others)  of  course 
justifies  the  statement  that  in  those  cases  conjugation  has 
caused  rejuvenescence,  as  maintained  by  Calkins.  Consis- 
tency with  the  facts  compels  one,  however,  to  say  that  while 
mating  sometimes  produces  rejuvenescence,  in  most  of  the 
individuals  that  mate,  it  produces,  in  the  majority  of  the 
known  species,  not  rejuvenescence,  but  its  opposite,  the  ani- 
mals being  less  vigorous  and  multiplying  less  rapidly  than 
would  have  been  the  case  if  they  had  not  mated. 

But  why  should  mating  produce  rejuvenescence  in  some 
cases  and  not  in  others?  In  other  words,  why  has  mating 
such  diverse  hereditary  results  in  different  cases  ?  This  ques- 
tion we  shall  take  up  in  our  next  lecture;  here  we  may  indi- 
cate merely  the  general  nature  of  the  answer  which  we  shall 
reach.  We  should  expect  conjugation  to  give  rejuvenescence 
just  as  endomixis  does,  since  in  it  too  the  old  macronucleus 
is  replaced  by  the  unused  micronucleus.  But  in  conjugation 
there  are  likewise  produced  new  combinations  of  the  parental 
characters.  Some  of  these  new  combinations  are  vigorous; 


164          Life  and  Death,  Heredity  and  Evolution 

others  are  weak.     In  the  former  rejuvenescence  appears;  in 
the  latter  it  does  not. 

What  conditions  bring  about  mating  in  these  organisms? 
It  is  of  interest  to  bring  the  facts  observed  on  this  point 
into  relation  with  those  we  have  above  set  forth  as  to  the 
results  of  mating.  Many  generations  pass  in  which  the  in- 
dividuals do  not  mate;  then  at  a  certain  time  some  or  many 
or  all  of  them  mate.  Is  this  the  result  of  the  internal 
changes  that  have  been  in  slow  progress,  so  that  conjugation 
occurs  when  a  condition  of  ripeness  or  of  need  for  it  has 
been  reached  ?  This  was  the  idea  held  by  many  who  believed 
the  life  of  these  creatures  to  go  in  cycles  of  youth  and  age; 
mating  occurs,  it  was  held,  in  a  certain  period  of  the  cycle. 
Or,  on  the  other  hand,  is  mating  rather  brought  on  by  certain 
external  conditions?  Much  study  has  been  devoted  to  these 
questions. 

In  Paramecium  and  many  other  infusoria  it  has  been  ob- 
served that  an  epidemic  of  mating  usually  occurs  when  a 
period  of  high  nutrition,  resulting  in  rapid  multiplication, 
is  followed  by  a  period  of  scarcity  of  food.  Artificial  cul- 
tures of  hay  or  other  vegetation  in  the  laboratory  often  go 
through  such  a  cycle;  at  first  bacteria  are  abundant  and 
the  infusoria  flourish  on  them ;  then  fermentative  changes  go 
so  far  that  the  appropriate  bacteria  are  scarce;  the  in- 
fusoria become  thin,  and  begin  to  mate.  It  is  easy  to  fur- 
nish these  conditions  if  from  a  flourishing  hay  culture  we 
remove  a  watch  glass  of  the  water  with  many  of  the  infusoria 
and  allow  it  to  stand,  without  any  of  the  vegetable  material, 
for  24  hours.  As  the  bacteria  become  scarce  the  infusoria 
conjugate.  This  method  has  been  used  practically  by  many 
investigators  in  order  to  obtain  matings  for  study. 

If  part  of  the  animals  are  kept  supplied  with  abundant 


Conditions  Inducing  Conjugation  165 

food,  while  the  others  are  subjected  to  a  scarcity,  the  latter 
conjugate,  while  the  former  do  not.  This  is  true  even  when 
all  the  individuals  are  derived  from  the  same  single  original 
parent.  It  is  clear  therefore  that  there  is  no  imperious  ne- 
cessity for  conjugation  at  a  particular  period  in  the  life 
history ;  and  that  a  period  of  scarcity  following  a  period  of 
abundance  will  induce  conjugation  when  it  would  otherwise 
not  occur.  In  some  of  the  writer's  experiments  the  offspring 
of  a  single  individual  were  divided  into  two  sets ;  one  set  was 
caused  in  this  way  to  go  through  conjugation  four  times 
in  succession  (the  mates  at  any  conjugation  being  the  off- 
spring of  the  mates  at  the  preceding  conjugation)  ;  while  the 
other  set  during  the  entire  period  did  not  conjugate  at  all. 
In  some  races  of  Paramecium  aurelia,  after  a  pair  had  mated 
their  descendants  in  the  fourth  generation  were  thus  caused 
to  mate  again;  while  in  other  members  of  the  same  stock 
hundreds  of  generations  passed  without  conjugation. 

Clearly  therefore  the  occurrence  of  conjugation  is  in  large 
measure  the  result  of  special  external  conditions.  This  mat- 
ter has  been  much  studied  of  late  by  Enriques  and  by  his 
pupil  Zweibaum.  They  have  found  that  conjugation  is 
favored  by  special  conditions  in  particular  species  of  in- 
fusoria; thus  in  Colpoda  steinii  conjugation  occurs  when  the 
layer  of  water  in  which  they  are  has  become  a  thin  film 
(Enriques  1907), — as  happens  just  before  a  pool  is  dried 
up  by  evaporation.  In  Cryptochilum  Enriques  (1910)  dis- 
covered that  certain  salts  greatly  favor  conjugation.  In 
Paramecium  caudatum  Zweibaum  (1912)  has  determined 
with  great  precision  the  conditions  that  induce  mating.  He 
finds  that  after  the  infusoria  have  been  subjected  to  a  period 
of  scarcity  of  food  for  five  to  six  weeks,  if  the  nutritive 
conditions  are  suddenly  changed  for  the  worse,  and  at  the 
same  time  certain  salts  are  present  in  proper  concentration, 


166         Life  and  Death,  Heredity  and  Evolution 

the  animals  will  always  conjugate.  The  salts  found  to  favor 
conjugation  were  the  compounds  of  sodium  and  other  metals 
with  chlorine,  bromine  and  other  halogens.  Aluminium 
chloride  was  found  to  be  the  most  favorable  of  those  studied. 

Thus  Zweibaum  and  Enriques  hold  that  the  environmental 
conditions,  past  or  present,  fully  determine  whether  conjuga- 
tion shall  occur.  It  is  true  that  of  two  stocks  side  by  side 
under  the  same  present  conditions,  one  may  conjugate,  the 
other  not;  but  this  in  their  opinion  is  due  to  the  fact  that 
one  has  been  subjected  to  a  long  period  of  scarcity  of  food, 
while  the  other  has  not.  That  is,  while  the  two  stocks  may 
indeed  at  a  given  time  differ  in  their  internal  conditions,  this 
difference  is  not  a  matter  of  diversity  in  the  life  cycle,  com- 
parable to  youth,  maturity  and  age,  but  is  merely  a  result  of 
the  different  external  conditions  under  which  they  have  been 
living.  The  occurrence  of  conjugation  is,  they  hold,  in  last 
analysis,  determined  by  external  conditions. 

There  is  certainly  a  large  measure  of  truth  in  this  conclu- 
sion, though  it  is  perhaps  not  yet  completely  established  in 
its  absolute  form.  The  question  may  be  asked  why  it  is 
necessary  that  the  period  of  scarcity  of  food  should  last  so 
long  as  five  to  six  weeks  before  it  induces  conjugation?  Does 
this  perhaps  indicate  that  a  certain  number  of  generations 
after  a  foregoing  conjugation  are  necessary  before  a  new 
mating  can  occur?  *  Zweibaum's  experiments  need  to  be 
repeated  in  such  a  way  that  after  one  conjugation  a  new 
culture  is  produced  from  an  ex-con jugant,  and  the  period  of 
time  determined  (or  if  possible  the  number  of  generations) 
that  must  necessarily  elapse  before  a  new  conjugation  can 
be  induced.  Zweibaum  did  not  determine  whether  an  inter- 
vening conjugation  does  away  with  the  accumulated  effects 
of  continued  scarcity  of  food,  so  that  the  organisms  must 

1This  question  has  already  been  raised  by  Erdmann  (1913). 


Conditions  Inducing  Conjugation  167 

again  be  subjected  to  five  or  six  weeks'  scarcity  before  they 
will  again  conjugate.  This  is  really  the  essential  point;  for 
if  this  turns  out  to  be  the  case,  then  evidently  the  length  of 
time  since  a  previous  conjugation  is  one  of  the  things  that 
determine  whether  conjugation  shall  now  occur. 

But  independently  of  this  doubtful  point,  it  is  clear  that 
mating  at  a  particular  period  is  not  required  independently 
of  the  outer  conditions,  for  Paramecium  aurelia  will  live  in- 
definitely (over  6000  generations)  without  conjugation 
(Woodruff),  yet  may  be  induced  to  conjugate  if  the  required 
outer  conditions  are  supplied  (Woodruff,  1914)  ;  and  in  some 
stocks  of  this  species  a  second  conjugation  may  be  induced 
in  the  fourth  generation  after  a  previous  conjugation  (Jen- 
nings, 1910,  p.  286).  Certainly  by  far  the  largest  part  is 
played  by  external  conditions  (past  or  present)  in  producing 
conjugation. 

The  conditions  under  which  mating  occurs  (sudden 
scarcity  of  food  and  the  like)  are  conditions  which  are  dis- 
tinctly unfavorable  to  the  life  of  the  organisms.  Some 
species  of  infusoria  respond  to  such  conditions  by  becoming 
encysted;  they  transform  into  a  small  sphere,  protected  by 
an  outer  coating;  and  in  this  state  they  can  withstand  con- 
ditions that  would  otherwise  destroy  them.  Some  other 
Protozoa  respond  by  first  conjugating,  then  encysting.  In 
others,  such  as  Paramecium,  there  is  only  conjugation,  with- 
out encystment.  But  as  we  have  seen,  conjugation  results 
in  the  production  of  many  diverse  stocks,  some  of  which  are 
more  resistant  to  given  conditions  than  others.  It  appears 
that  some  of  the  stocks  so  produced  may  be  able  to  survive 
the  unfavorable  conditions  which  induced  conjugation,  al- 
though (as  observation  shows)  most  of  them  die  out  if  the 
conditions  are  not  altered  for  the  better.  Later  generations 
would  therefore  all  be  derived  from  the  most  vigorous  and 


168          Life  and  Death,  Heredity  and  Evolution 

resistant  stocks  resulting  from  the  new  combinations  formed 
in  mating.  There  is  ground  for  believing  that  in  nature 
this  process  occurs  on  a  large  scale. 

Looking  back  over  what  has  been  found  out  as  to  the 
effects  of  mating,  the  general  picture  is  as  follows:  It  has 
been  shown  that  infusoria  may  live  and  multiply  indefinitely 
without  conjugation  (Woodruff,  Enriques).  It  has  been 
shown  that  at  intervals  the  old  active  macronucleus  is  re- 
placed by  a  part  of  the  reserve  micronucleus.  These  things 
demonstrate  that  the  mating  process  (as  distinguished  from 
the  replacement  process)  is  not  necessary  for  continued  life 
and  vigor.  They  appear  to  disprove  any  theory  of  sexuality 
that  maintains  that  there  must  for  continued  life  be  a 
periodic  reunion  of  two  substances,  male  and  female,  which 
inevitably  become  separated  as  a  result  of  life  and  develop- 
ment. Rejuvenescence  is  through  the  replacement  of  used 
parts  by  unused  ones,  and  this  occurs  without  mating,  al- 
though it  may  occur  at  mating  also.  The  distinctive  con- 
tribution of  the  mating  itself  is  something  else. 

Investigation  shows  that  mating  produces  two  very  strik- 
ing results :  ( 1 )  It  causes  the  offspring  of  the  two  individu- 
als that  have  conjugated  to  become  more  alike;  it  produces 
biparental  inheritance.  (2)  It  causes  the  different  families 
produced  by  different  pairs  to  be  hereditarily  diverse  in  many 
respects ;  and  this  even  when  all  the  parents  come  from  a 
single  ancestor  and  are  hereditarily  alike. 

Do  we  find  anything  of  this  sort  elsewhere  in  organisms? 
Consideration  brings  to  light  the  fact  that  this  is  precisely 
what  results  from  mating  in  higher  organisms;  we  call  the 
detailed  working  out  of  these  results  Mendelian  heredity. 
In  heredity  in  higher  organisms,  the  offspring  produced  by 
any  pair  resemble  each  other  more  than  they  do  other  in- 
dividuals; they  show  biparental  inheritance.  Furthermore, 


Results  of  Mat'mg  169 

the  offspring  produced  by  the  different  germ  cells  of  even 
the  same  pair  of  parents  are  hereditarily  diverse;  a  single 
pair  of  parents  may,  in  plants  or  certain  animals,  produce 
in  Mendelian  inheritance  hundreds  of  hereditarily  different 
kinds  of  offspring.  The  only  reason  why  this  may  not 
occur  in  the  highest  animals  and  man  is  that  in  these  cases 
relatively  few  of  the  possible  combinations  develop,  since 
but  few  offspring  are  produced. 

In  these  respects,  therefore,  mating  does  the  same  thing 
in  the  Protozoa  that  it  does  in  the  higher  organisms.  In 
both  it  brings  biparental  inheritance  and  the  production  of 
hereditarily  diverse  stocks. 


VII 


How  Does  Mating  Bring  About  Both  Biparental  Inherit- 
ance and  Diversity  in  Hereditary  Characters?  What  Effect 
Has  Mating  on  the  Stock  as  a  Whole?  Does  It  Increase 
Variation?  Does  It  Decrease  Variation?  What  Is  Its 
Relation  to  Evolution? 

T  N  our  last  chapter  we  showed  that  mating  produces  bi- 
parental  inheritance,  as  well  as  diversity  of  inherited 
characteristics,  in  lower  as  well  as  in  higher  organisms. 
How  are  these  results  brought  about?  How  does  it  happen 
that  the  offspring  of  the  two  members  of  a  pair  on  the  whole 
resemble  each  other,  yet  are  hereditarily  diverse? 

The  main  outlines  of  the  way  this  is  brought  about  are 
well  known.  Each  parent  hands  on  bodily  to  the  offspring, 
through  the  germ  cells,  certain  packets  of  chemicals.  Since 
these  are  directly  transmitted  from  parent  to  offspring, 
while  the  later  characters  are  secondarily  derived  from 
them,  we  may  call  these  packets  of  chemicals  the  primary 
hereditary  characters.  These  packets  are  present  in  each 
animal  in  a  certain  definite  number,  stored  within  the 
nucleus;  they  are  called  chromosomes  (see  Figures  29,  31, 
32).  Individuals  which  get  different  sets  of  packets  from 
their  parents  develop  differently  even  under  the  same  outer 
conditions ;  that  is,  they  show  different  hereditary  char- 
acteristics. 

This  arrangement  of  the  primary  hereditary  characters — 
the  chemicals  that  determine  the  hereditary  peculiarities — 
170 


The  Primary  Hereditary  Characters  171 

into  packets  is  fundamental  for  an  understanding  of  how 
heredity  occurs ;  it  is  this  that  directly  brings  about  all  the 
peculiar  phenomena  that  are  called  Mendelian  inheritance. 
And  this  is  a  typical  illustration  of  the  part  played  by 
structure  and  arrangement  in  organisms;  it  demonstrates 
that  a  chemical  study  alone,  omitting  arrangement  of  the 
chemicals,  can  never  lead  to  comprehension  of  what  occurs. 
The  point  is  that  when  two  or  more  chemicals  are  in  a  cer- 
tain space,  it  makes  all  the  difference  in  the  world  as  to 
what  happens,  whether  the  two  substances  are  in  separate 
bottles,  or  merely  poured  together.  To  neglect  this  fact 
in  organisms  is  as  fatal  to  understanding  them  as  it  would 
be  to  try  to  comprehend  what  occurs  in  a  chemical  labor- 
atory without  realizing  that  the  different  chemicals  are 
kept  in  separate  containers.  The  nucleus  of  the  cell  is  a 
chemical  laboratory  containing  diverse  chemicals  in  separate 
packets.  At  times  substances  come  out  of  these  packets, 
intermingle,  and  therefore  react.  It  is  their  reactions  with 
each  other,  and  with  external  conditions,  in  an  orderly  way, 
that  bring  about  growth  and  the  development  into  a  struc- 
ture with  diverse  organs.  The  packets  are  shifted  about  and 
distributed  in  certain  ways  at  the  time  of  mating  and  fer- 
tilization, and  it  is  the  rules  of  their  distribution  that  are 
what  we  call  the  rules  or  laws  of  inheritance. 

We  know  that  in  any  organism  these  packets  of  chemicals 
are  present  in  a  definite  arrangement.  We  know  that  each 
larger  packet  or  chromosome  is  a  chain  or  group  of  con- 
nected small  packets  (Figure  29,  E,  F),  and  that  the  dif- 
ferent chromosomes  present  in  a  nucleus  are  diverse.  Their 
number  is  definite  in  any  individual,  and  they  are  so  con- 
stituted as  to  form  a  set  of  diverse  pairs  (Figure  29;  Figure 
45  B);  (though  sometimes  there  is  a  single  package  or 
chromosome  that  is  not  paired  with  another). 


17£          Life  and  Death,  Heredity  and  Evolution 

When  mating  is  to  occur  we  know  that  these  paired 
packages  of  each  nucleus  separate  into  two  groups,  each 
group  containing  one  member  of  each  pair  (Figure  45,  B, 
C,  D).  These  two  groups  are  then  separated  by  cell  divi- 
sion into  different  cells.  Each  of  these  cells  therefore  con- 
tains a  group  with  half  the  number  of  packages  that  were 
present  in  the  parent  nucleus.  It  is  such  cells  with  half 
the  original  number  of  packages  or  chromosomes  in  their 
nuclei  that  form  the  germ  cells, — the  two  cells  that  are  to 


* 


Figure  45.  The  separation  of  the  two  groups  of  paired  chromosomes 
into  different  germ  cells,  in  the  insect  Nezara  hilaris,  after  Wilson, 
1911.  A,  the  14  chromosomes  in  a  single  cell,  before  the  germ  cells 
are  formed.  B,  the  14  gathered  into  7  pairs.  C,  the  members  of  the 
seven  pairs  separating  as  the  division  to  form  the  germ  cells  occurs. 
D,  the  two  groups  of  7  chromosomes  each,  in  different  germ  cells, 
formed  by  the  separation  of  the  14  shown  in  A  and  B. 

unite  in  mating  (Figure  45,  D).  After  the  two  half  nuclei 
have  united,  of  course  the  original  number  of  chromosomes 
is  restored. 

As  before  remarked,  we  know  that  the  different  chromo- 
somal packages  present  in  a  nucleus  are  diverse.  The 
evidence  for  this  is  complete,  but  cannot  be  given  here. 
Now,  when  these  diverse  packages  separate  into  two  half 
groups,  different  half  groups  are  formed  in  different  cases, 
depending  on  which  member  of  any  given  pair  goes  into  a 
given  group.  In  this  way  a  great  number  of  diverse  com- 


Distribution  of  the  Primary  Hereditary  Characters     173 

binations  are  formed  in  the  half-groups  derived  from  dif- 
ferent nuclei,  even  of  the  same  parent. 

This  diversity  of  the  combinations  of  the  primary  hered- 
itary characters  in  the  different  half  nuclei  is  the  essential 
point  in  understanding  the  way  the  later  hereditary  char- 
acters are  distributed,  so  that  it  will  be  best  to  illustrate 
how  it  comes  about.  Suppose  we  represent  the  chromosomal 
packages  in  the  nuclei  of  a  particular  animal  by  letters  of 
the  alphabet.  We  will  indicate  the  diversities  by  giving 
a  different  letter  to  each  chromosomal  packet,  and  to  the 
two  members  of  a  given  pair  we  will  give  a  capital  letter 
and  a  small  letter  respectively.  To  illustrate  the  principles 
in  simple  form,  we  will  suppose  that  there  are  but  four  pairs 
of  chromosomal  packages  in  each  nucleus.  That  is,  the 
chromosomes  of  each  nucleus  would  be  represented  as  fol- 
lows: 

A  B  C  D 
abed 

Now  in  each  parental  nucleus  of  this  kind  the  chromosomes 
separate  into  two  groups,  one  member  of  each  pair  in  each 
group.  But  either  member  of  any  pair  can  go  into  either 
group.  That  is,  from  one  nucleus  the  two  groups  formed 
may  be  A  B  C  D  and  abed;  from  another  nucleus  they 
are  A  B  c  D  and  a  b  C  d ;  from  another  A  b  C  d  and  a  B  c  D, 
and  so  on.  The  number  of  different  combinations  from 
four  pairs  is  16,  and  each  occurs  as  frequently  as  any  other.1 

So  from  a  number  of  parental  nuclei,  all  having  the  same 
combination  of  packages,  a  large  number  of  different  com- 
binations will  be  formed  in  the  half  nuclei.2 

1  These  facts,  fundamental  for  the  understanding  of  the  rules  of  in- 
heritance, have  recently  been  directly  demonstrated  for  certain  higher 
organisms,  by  Carothers  (1917). 

'The  number  of  diverse  combinations  possible  in  the  half  nuclei 
formed  from  nuclei  all  of  the  same  kind  is  2",  if  n  is  the  number  of 
diverse  pairs  of  chromosomes  present  in  the  original  nuclei. 


174          Life  and  Death,  Heredity  and  Evolution 

The  offspring  are  finally  produced  by  the  mating  of  two 
of  these  half  cells  (or  germ  cells),  each  containing  a  half 
nucleus.  A  nucleus  with  any  combination  of  the  chromo- 
somal packages  may  unite  with  any  other.  Thus  we  shall 
get  in  the  case  imagined  such  combinations  as 

ABcD  aBcd 

aBCd  or  aBCd    and  the  like. 

The  total  number  of  different  combinations  produced  when 
these  were  originally  four  pairs  of  different  chromosomal 
packets  is  81. 3 

If  the  number  of  chromosomal  pairs  is  greater,  the  number 
of  combinations  produced  by  mating  is  greatly  increased. 
For  each  additional  pair  of  chromosomes  the  number  of 
possible  combinations  is  multiplied  by  3.  Where  there 
are  24  pairs  of  chromosomes,  as  apparently  in  man,  the 
number  of  possible  diverse  combinations  mounts  far  up  into 
the  trillions.  , 

Each  one  of  these  combinations  of  chromosomal  packets 
gives  a  different  result  in  heredity;  a  different  set  of 
hereditary  characters.  The  result  is  that  the  progeny  pro- 
duced by  the  different  germ  cells  differ  from  each  other,  and 
it  becomes  impossible  to  predict  from  the  characteristics  of 
the  parent  what  will  be  the  characteristics  of  particular  off- 
spring, for  many  diverse  kinds  of  offspring  can  be  produced 
by  the  same  pair  of  parents. 

These  processes  are  best  known  in  higher  organisms.  But 
study  shows  that  the  same  things  occur  in  the  Protozoa. 
These  matters  are  extremely  difficult  to  work  out  in  these 
minute  creatures,  and  an  immense  amount  of  work  remains 
to  be  done  before  we  shall  know  with  full  details  what  happens 

•If  there  were  »  pairs  of  chromosomes  present  in  the  original  nuclei, 
then  by  the  formation  and  mating  of  germ  cells  in  the  way  described, 
the  number  of  different  combinations  producible  is  3". 


Distribution  of  the  Primary  Hereditary  Characters    175 

in  these  organisms.  But  we  find  that  in  these,  as  in  higher 
animals,  there  are  diverse  packets  of  chemicals,  which  are 
directly  transmitted  from  parent  to  offspring,  so  that  they 
constitute  the  primary  hereditary  characters.  In  some  of 
the  Protozoa  these  chromosomes  are  extremely  minute  and 
numerous ;  this  is  the  case  in  the  infusorian  Paramecium 
caudatum  (Figure  49).  In  others  they  are  larger  and 
present  in  smaller  numbers,  appearing  much  as  they  do  in 
higher  organisms  (Figure  46).  The  primary  hereditary 
characters  or  chromosomes  are  shown  for  a  number  of 
Protozoa  in  figures  46  to  50. 

In  preparation  for  mating,  and  in  mating  itself,  these 
chromosomes  undergo  the  same  process  of  reduction  in 
number  and  recombination  into  new  groups  that  occurs  in 
higher  organisms.  In  figures  46  to  50  is  shown  what  occurs 
in  some  of  the  groups  of  Protozoa. 

Examine  for  example  figure  46,  which  shows  the  process  in 
a  protozoan  belonging  to  the  Gregarinidse,  and  parasitic  in 
the  earthworm, — as  described  by  Mulsow  (1911).  The 
chromosomal  packets  are  in  the  form  of  eight  long  threads 
(Figure  46,  A)  much  resembling  the  chromosomes  of  many 
higher  animals.  In  ordinary  multiplication  by  fission  each 
of  these  chromosomes  splits  lengthwise  (Figure  46,  B),  and 
half  of  each  goes  to  each  of  the  two  offspring  (C).  In  the 
two  parents  before  conjugation  there  are  as  usual  eight  of 
these  chromosomes,  which  become  arranged  side  by  side  in 
pairs,  as  occurs  in  higher  organisms.  This  grouping  into 
four  pairs  is  partly  seen  in  figure  46,  D  and  E.  Now  in 
the  early  stages  of  mating,  a  division  of  the  nucleus  occurs 
at  which  one  member  of  each  pair  goes  to  one  of  the  daughter 
nuclei,  one  to  the  other  (F,  G,  H).  That  is,  each  of  the 
two  nuclei  produced  now  receives  four  entire  chromosomes,  in 
place  of  eight.  Then  in  the  mating,  two  such  nuclei,  each 


176         Life  and  Death,  Heredity  and  Evolution 


.^/•--•;  ,.     siiii:ii- 

tesOsg^   |4]«jil] 


?*.  jf*^V^'i«n^i 


Reduction  in  the  Protozoa  177 

with  four  chromosomes,  unite,  forming  a  new  nucleus  with 
eight  chromosomes.  In  this  entire  process  of  reducing  the 
number  to  four  and  then,  by  mating,  restoring  it  to  eight, 
of  course  many  different  combinations  of  the  chromosomes 
may  arise  in  the  different  resulting  individuals,  in  the  way 
already  set  forth.  The  number  of  possible  diverse  com- 
binations in  this  case  with  four  pairs  of  chromosomes,  is, 
as  we  have  seen,  81. 

Reduction  is  better  known  in  the  ciliate  infusoria  than 
in  any  other  group  of  Protozoa.  To  understand  what  hap- 
pens, one  must  recall  the  fact  that  at  the  beginning  of  mat- 
ing there  are  three  successive  divisions  of  the  micronucleus, 
the  third  one  producing  the  migratory  and  stationary  half 
nuclei.  These  are  indicated  in  Figure  35.  These  three 
divisions  are  commonly  spoken  of  as  the  first,  second  and 
third  maturation  divisions;  we  shall  employ  these  designa- 
tions. • 

In  the  infusorian  Didinium  nasutum  (Figure  47),  accord- 
ing to  Prandtl  (1906),  there  are  16  minute  chromosomes 
(A).  In  the  first  of  the  three  maturation  divisions  each  of 
these  16  chromosomes  divides  into  2,  so  that  the  resulting 
two  micronuclei  still  have  16  chromosomes  (Figure  47,  B). 
But  in  the  second  division,  the  16  chromosomes  merely  sep- 
arate into  two  groups  of  8,  one  group  going  to  each  of  the 
two  resulting  micronuclei  (C,  D,  E,  F).  In  the  third 
division  (G,  H,  I)  each  of  the  8  chromosomes  present  divides 
into  two,  so  that  each  of  the  two  half  nuclei  now  has  8 
chromosomes.  Now  the  migratory  half  nucleus  from  one 
mate  passes  over  and  unites  with  the  stationary  half  nucleus 
of  the  other  (Figure  47,  J,  K,  L),  so  that  the  resulting 
complete  nucleus  now  has  16  chromosomes.  In  the  later 
divisions  of  this  complete  nucleus,  each  of  the  16  chromo- 
somes divides  (M),  so  that  all  the  nuclei  of  later  generations 


178          Life  and  Death,  Heredity  and  Evolution 
A  ^-v          C  _          D  .  E 


FIG.  47. 
(For  description  fee  opposite  page) 


Conjugation  m  Didinium  Nasutum  179 


Figure  47.  Reduction  of  the  number  of  chromosomes,  and  other 
processes  in  the  nuclei,  at  conjugation  in  Didinium  nasutum.  After 
Prandtl,  1906. 

A  and  B,  the  'first  of  the  three  divisions  of  the  micronuclei  ("first 
maturation  division").  In  A,  a  spindle  has  been  formed  with  16  chromo- 
somes; in  B,  each  chromosome  has  divided,  so  that  two  groups  of  16 
are  present;  one  group  to  go  to  each  of  the  two  resulting  micronuclei. 

C,  D,  E  and  F,  the  second  division  (the  "reducing"  division).  C,  16 
chromosomes;  spindle  forming  for  division.  D,  the  16  chromosomes 
beginning  to  separate  into  2  groups.  E,  the  16  chromosomes  have  sep- 
arated into  2  groups  of  eight  each,  each  going  to  one  of  the  two  result- 
ing micronuclei.  F,  the  two  resulting  micronuclei,  each  with  8  chromo- 
somes; still  united  by  a  connecting  strand  from  the  spindle. 

G,  H  and  I,  the  third  division,  which  forms  from  a  single  nucleus 
the  migratory  and  the  stationary  nucleus.  G,  each  of  the  8  chromosomes 
dividing.  H,  the  two  groups  of  8  chromosomes  widely  separated,  to 
pass  into  the  two  resulting  nuclei.  I,  the  migratory  nucleus  (above) 
and  the  stationary  nucleus  (below)  still  joined  by  a  connecting  strand. 
The  stationary  nucleus  already  considerably  larger  than  the  migratory 
nucleus. 

J,  the  migratory  nucleus  passing  through  the  membrane  that  sep- 
arates the  two  mated  animals,  into  the  other  individual. 

K,  union  of  the  migratory  and  the  stationary  nuclei.  The  latter 
(above)  is  much  larger  than  the  former.  L,  the  two  nuclei  almost 
completely  united. 

M,  the  first  division  of  the  nucleus  formed  by  the  union  of  the  migra- 
tory and  stationary  nuclei.  The  union  is  not  quite  complete,  so  that 
at  the  right  end  two  spindles  can  be  seen.  Each  of  the  16  chromosomes 
has  divided  into  two,  so  that  two  groups  of  16  are  now  present. 


180          Life  and  Death,  Heredity  and  Evolution 


have  16  chromosomes, — until  another  reduction  occurs 
preparatory  to  another  mating. 

In  the  infusorian  Anoplophrya  branchiarum,  which  is  a 
parasite  in  the  blood  of  the  fresh  water  crustacean  Gam- 
marus,  the  number  of  chromosomes  is  6.  These  are  reduced 
to  3  before  the  mating  (Figure  48)  ;  by  the  union  of  two 
at  mating  the  number  6  is  restored  (Collin,  1909). 

In  Opercularia  coarctata,  a  relative  of  Vorticella,  accord- 
ing to  Enriques  (1907),  the  unreduced  number  is  16;  the 


Figure  48.  Conjugation  and  reduction  in  the  number  of  chromosomes 
in  the  infusorian  Anoplophrya  branchiarum,  after  Collin,  1909.  A, 
the  two  micronuclei  (after  the  first  maturation  division)  have  each  6 
small  chromosomes.  B,  each  micronucleus  dividing  anew,  showing  the 
separation  of  the  group  of  6  into  two  groups  of  3,  one  at  each  end 
of  the  spindle. 

reduced  number  8.  The  reduction  occurs  at  the  second  of 
the  three  maturation  divisions;  and  the  number  16  is  re- 
stored at  mating.  In  the  infusorian  Chilodon  uncinatus, 
according  to  the  same  author  (Enriques,  1908),  the  number 
of  chromosomes  before  reduction  is  4.  At  the  second 
maturation  division,  these  are  reduced  to  2,  in  the  usual  way. 
Mating  restores  the  original  number  4. 

In  Carchesium,  according  to  Popoff  (1908),  the  micro- 
nuclei  have,  before  the  time  of  mating,  16  chromosomes. 
At  the  first  maturation  division  8  of  these  go  into  one  of  the 
resulting  half  nuclei,  8  into  the  other.  At  the  second  and 
third  divisions  each  of  these  8  chromosomes  divides  into  two, 


Reduction  m  the  Protozoa  181 

so  that  finally  the  two  half  nuclei  that  mate  have  each  8 
chromosomes.  The  original  number,  16,  is  restored  by  the 
mating. 

In  Opalina  intestinalis,  a  large  infusorian  parasitic  in  the 
alimentary  canal  of  amphibians,  the  number  of  chromosomes 
during  ordinary  reproduction  by  fission  is  8.  In  the  spring 
there  appear  small  animals,  which  divide  several  times,  then 
encyst;  when  they  come  out  of  the  cysts  they  mate,  two 
individuals  completely  uniting.  These  small  individuals 
before  mating  have  but  4  chromosomes  in  place  of  8.  How 
the  reduction  is  brought  about  is  not  known  (Metcalf,  1909). 

In  the  two  common  species  of  Paramecium,  aurelia  and 
caudatum,  the  nuclear  processes  at  mating  appear  to  differ 
considerably.  They  are  best  known  in  Paramecium  cau- 
datum, through  the  work  of  Calkins  and  Cull  (1907).  In 
this  animal  the  matter  is  greatly  complicated  by  the  fact 
that  a  very  large  number  of  chromosomes  is  present  (Figure 
4$).  The  number  is  so  great  that  they  cannot  be  counted, 
but  Calkins  and  Cull  estimate  them  at  about  165. 

In  individuals  beginning  mating,  the  chromosomes  appear 
as  double  rods  (Figure  49,  A).  Calkins  and  Cull  suspect 
that  this  is  due  to  the  pairing  of  two  chromosomes,  such  as 
we  saw  in  Monocystis  (Figure  46).  At  both  the  first  and 
second  maturation  divisions  these  double  chromosomes  split 
lengthwise.  One  of  these  divisions  therefore  apparently 
separates  the  paired  chromosomes  (reducing  the  number  to 
half  in  each  resulting  half  nucleus);  the  other  divides  each 
chromosome  lengthwise.  The  reduced  number  is  apparently 
therefore  present  in  the  micronuclei  before  the  third  division. 
This  third  division  takes  place  in  a  very  different  way  from 
the  other  two.  The  chromosomes,  instead  of  being  long 
threads,  fall  into  strings  of  granules ;  and  each  of  these 
strings  is  broken  transversely,  at  about  its  middle  (G). 


182          Life  and  Death,  Heredity  and  Evolution 


Figure  49.  The  chromosomes  and  the  divisions  preparatory  to  mating 
in  Paramecium  caudatum,  after  Calkins  and  Cull,  1907.  A,  B,  C, 
first  of  the  three  maturation  divisions.  A,  the  numerous  chromosomes 
united  in  pairs  lengthwise  (or  split?).  B,  the  two  chromosomes  (or 
halves?)  separating  lengthwise.  C,  the  two  chromosomes  (or  halves) 
almost  separated;  two  groups  forming.  D,  one  of  the  two  nuclei 
resulting  from  the  first  maturation  division.  E,  F,  second  maturation 
division.  E,  the  two  halves  of  the  chromosomes  pulling  apart.  F,  the 
two  groups  separated;  the  two  new  micronuclei  united  by  a  narrowed 
connecting  zone.  G,  third  division  (that  producing  the  migratory  and 
stationary  half  nuclei).  The  chromosomes  formed  of  rows  of  particles; 
the  rows  have  broken  in  the  middle  and  the  halves  are  separating.  H, 
union  of  migratory  and  stationary  half  nuclei,  in  the  two  individuals. 
The  oblique  line  is  the  surface  of  separation  of  the  two  mates.  In  each 
individual  the  migratory  half  nucleus  is  the  smaller  one. 


Thus  at  this  division  each  of  the  two  half  nuclei  formed 
receives  a  half  of  each  of  the  chromosomes  (then  present  in 
the  reduced  number).  The  original  number  would  of  course 
be  restored  by  the  mating  of  the  migratory  -and  the  sta- 
tionary nuclei  (H). 

Reduction  is  not  so  well  known  in  the  other  groups  of 


Reduction  in  the  Protozoa 


183 


Protozoa  as  in  the  ciliate  infusoria.  In  the  gregarine 
Monocystis  however  we  find  a  particularly  beautiful  example 
(Figure  46).  In  the  flagellates,  Schaudinn  (1904)  and 
Prowazek  (1904)  described  a  reduction  from  8  chromosomes 
to  4,  in  a  number  of  species  (Trypanosoma  noctuae,  T. 
lewisi,  T.  brucei,  and  Herpetomonas).  The  accuracy  of 
these  accounts  has  been  called  in  question. 

Bott  (1907)  has  described  the  reduction  of  the  number  of 
chromosomes    in    the    rhizopod    Pelomyxa,    an    animal    re- 


Figure  50.  Reduction  of  the  number  of  chromosomes  before  mating 
in  the  rhizopod  Pelomyxa,  after  Bott,  1907.  A,  the  eight  oval  chromo- 
somes of  the  parent.  B,  first  of  the  two  maturation  divisions;  the  8 
chromosomes  separating  into  two  groups  of  4.  C,  the  second  matura- 
tion division;  in  the  spindle  to  the  left  each  of  the  4  chromosomes  (of 
which  but  3  are  in  view)  is  dividing  into  two. 


sembling  a  large  amoeba.  The  single  animal  contains  many 
nuclei.  At  times  these  go  through  two  divisions  in  succes- 
sion, which  may  be  called  the  maturation  divisions  (see 
Figure  50).  At  first  there  are  eight  oval  chromosomes 
(A).  At  the  first  division  (B),  four  of  these  go  into  one 
of  the  resulting  cells,  four  into  the  other;  the  number  is 
thus  reduced.  At  the  second  division  each  of  the  four 
chromosomes  is  divided  (C),  so  that  all  the  nuclei  produced 
have  four  chromosomes.  Later  each  of  these  nuclei,  along 
with  a  little  cytoplasm,  separates  from  the  body  of  the 


184«         Life  and  Death,  Heredity  and  Evolution 

mother,  forming  a  free  cell  or  gamete.  When  two  of  these 
gametes  meet,  they  unite;  thus  the  original  number  of 
chromosomes  is  restored. 

Only  a  few  of  the  Protozoa  have  been  examined  with  suf- 
ficient thoroughness  to  reveal  this  process  of  reduction  and 
recombination  in  the  chromosomes,  and  in  some  the  chromo- 
somes are  so  numerous,  minute  and  crowded  that  just  what 
occurs  cannot  be  directly  determined.  But  the  cases  al- 
ready worked  out,  scattered  as  they  are  through  the  dif- 
ferent classes  of  the  group,  show  that  the  process  is  one 
of  general  occurrence,  here  as  in  the  higher  organisms ;  they 
make  it  possible  to  recognize  the  occurrence  of  reduction 
even  when  the  chromosomes  cannot  be  counted.  Just  be- 
fore mating,  the  nuclei,  both  in  the  Protozoa  and  in  higher 
organisms,  go  through  the  two  successive  divisions  (in  the 
infusoria,  owing  to  special  conditions,  three),  known  as  the 
maturation  divisions.  It  is  in  one  of  these  as  a  rule  that 
the  reduction  in  number  occurs,  through  the  distribution  of 
half  the  chromosomes  to  one  nucleus,  half  to  the  other.  In 
most  cases  these  two  (or  three)  divisions  are  distinguishable 
from  all  others  by  marked  peculiarities  connected  with  the 
reducing  process;  and  these  make  it  possible  to  recognize 
the  reducing  divisions  even  when  the  number  of  chromosomes 
cannot  be  counted.  Whenever  two  (or  three)  peculiar  divi- 
sions occur  in  rapid  succession  just  before  the  nuclei  are 
ready  to  mate,  we  may  be  practically  certain  that  in  these 
the  reduction  in  the  number  of  chromosomes  has  occurred. 
Such  divisions  we  saw  in  the  cases  of  autogamy  (page  134 
and  Figure  41 );  in  our  examination  of  the  preparations 
for  mating  in  the  infusoria  (Figures  46  to  50)  ;  they  occur 
indeed  almost  universally  in  preparation  for  mating.  All 
such  divisions  indicate  a  process  of  reduction  in  number 


Recombinations  of  the  Primary  Hereditary  Characters     185 

of  chromosomes,  with  resulting  formation  of  a  new  combina- 
tion through  mating. 

For  the  Protozoa  it  is  clear  therefore  that  so  far  as  the 
primary  hereditary  characters — the  chromosomes — are  con- 
cerned, mating  is  a  process  of  producing  new  combinations 
of  hereditary  characters.  In  higher  organisms  we  know 
that  these  primary  hereditary  characters  are  what  determine 
also  the  later  or  secondary  hereditary  characters, — those 
that  appear  in  the  developed  body.  There  can  be  no  doubt 
that  the  same  is  true  of  the  Protozoa.  Everything  indicates 
that  in  these  respects  mating  in  the  Protozoa  is  the  same 
sort  of  thing  that  it  is  in  higher  organisms,  and  that  when 
the  matter  is  fully  studied  it  will  be  found  to  produce  the 
same  kind  of  results.  Mating  may  be  defined  as  the  process 
of  producing  new  groups  of  hereditary  characters,  primary 
and  secondary,  by  combining  diverse  half  groups  from  dif- 
ferent nuclei. 

But  what  shall  be  said  from  this  point  of  view  of  the  cases 
in  which  the  two  half  nuclei  are  produced  from  a  single  one, 
and  these  two  later  unite  in  mating,  as  in  the  numerous  cases 
of  autogamy  (Figure  41,  etc.)?  These  cases  form  a  diffi- 
culty for  almost  any  other  way  of  looking  at  the  matter,  but 
not  for  this  one.  For  new  combinations  of  the  primary 
hereditary  characters  are  formed  likewise  when  the  mating  is 
between  two  nuclei  that  are  recent  products  of  a  single  one. 

In  all  cases  of  such  autogamy,  a  fact  is  observed  which 
becomes  of  the  greatest  significance.  After  a  single  nucleus 
has  divided  into  two,  these  two  do  not  reunite  at  once,  but 
there  are  always  one  or  two  intervening  divisions.  And  it 
is  these  intervening  divisions  that  bring  about  the  reduction 
in  number  of  the  chromosomal  packets,  with  the  consequent 
formation  of  new  combinations  of  the  primary  hereditary 


186         Life  and  Death,  Heredity  and  Evolution 

characters  in  mating.  Thus,  in  Figure  41,  we  see  that  after 
the  separation  of  the  original  nucleus  into  two,  each  of  these 
two  divides  twice,  giving  off  two  very  small  nuclei,  which 
are  absorbed  and  disappear. 

It  is  worth  while  to  notice  just  how  new  combinations  are 
formed  in  these  cases  in  which  the  two  nuclei  that  mate 
originally  came  from  the  same  single  nucleus.  Let  us  sup- 
pose that  the  original  single  nucleus  had  four  pairs  of  the 
chromosomal  packets ;  these  we  may  designate  as  follows : 

A  B  C  D 
abed 

Now  when  this  nucleus  divides  into  two,  the  division  is  of 
the  usual  sort,  in  which  each  single  packet  divides  into  two 
like  itself,  so  that  each  of  the  two  nuclei  produced  has  the 
same  set  of  chromosomal  packets  that  its  parent  had. 

But  now  each  of  these  two  goes  through  the  "reducing 
division,"  in  which  the  set  of  eight  divides  into  two  sets  of 
four  each, — one  member  of  each  pair  going  to  each  resulting 
set  of  four.  Then  evidently  many  different  combinations 
may  be  formed,  depending  on  how  the  members  are  dis- 
tributed. In  one  nucleus  the  group  of  four  will  be  A  B  C  D, 
in  another  A  B  C  d,  in  another  A  b  c  D,  in  another  a  b  C  d, 
and  so  on  (16  different  combinations  are  possible).  Now 
two  of  these  combinations  of  four  unite.  It  is  practically 
certain  that  they  will  have  different  combinations ;  let  us 
suppose  that  one  contained  the  combination  A  b  c  D,  the 
other  the  combination  a  b  C  d;  then  the  nucleus  formed  by 
their  union  will  show  the  combination 

A  b  c  D 

a  b  C  d 

That  is,  from  a  nucleus  showing  the  combination 

A  B  C  D 
abed 


Recombinations  of  the  Primary  Hereditary  Characters     187 

we  have  by  division,  reduction,  and  reunion  obtained  a 
totally  different  combination. 

In  other  cases,  nuclei  with  the  same  original  combination 
will  give  still  different  results ;  there  are  81  diverse  resultant 
combinations  that  may  be  produced  in  this  way  from  a  sin- 
gle nucleus  having  the  combination  of  packets  supposed 
above. 

Since  the  facts  as  to  the  recombination  and  distribution  of 
these  primary  hereditary  characters  are  the  same  in  Pro- 
tozoa and  in  higher  organisms,  we  may  expect  them  to  pro- 
duce the  same  results.  That  is,  we  may  expect  to  find  Men- 
delian  inheritance  in  Protozoa,  when  the  facts  are  fully  stud- 
ied. Mendelian  inheritance  is  nothing  more  nor  less  than  a 
recombination  and  distribution  of  the  secondary  hereditary 
characters  in  the  manner  that  the  primary  hereditary  char- 
acters are  recombined  and  distributed, — without  doubt  in 
consequence  of  this  recombination  and  distribution  of  the 
primary  characters. 

There  is,  of  course,  no  reason  to  expect  such  a  Mendelian 
distribution  of  inherited  characters  among  the  progeny  of  a 
single  individual  that  divides  after  conjugation.  All  such 
progeny  will  probably  be  alike,  save  for  any  accidental  vari- 
ations in  the  splitting  of  the  chromosomal  packets ;  this,  as 
we  have  seen,  observation  shows  to  be  the  case.  But  if  the 
progeny  are  examined  from  a  large  number  of  pairs  coming 
from  two  diverse  races  that  have  been  induced  to  conjugate, 
we  may  expect  to  find  among  these  the  typical  Mendelian 
distribution  of  characters. 

This  may  be  illustrated  most  directly  by  the  inheritance 
of  the  primary  characters  (the  chromosomes)  ;  we  know  that 
the  secondary  hereditary  characters  follow  the  primary  ones. 

Let  us  suppose,  for  example,  that  in  the  two  races  a  pair 
of  chromosomes  differ ;  we  will  say  that  they  are  black  in  one 


188         Life  and  Death,  Heredity  and  Evolution 

race,  white  in  the  other  (see  Figure  51,  P).  Now  we  know 
that  in  each  individual  before  mating,  one  chromosome  of 
each  pair  is  gotten  rid  of,  the  half  nucleus  that  remains  con- 
taining but  a  single  chromosome  of  this  pair;  and  that  this 
half  nucleus  divides  to  form  the  migratory  and  stationary 
half  nuclei  of  that  individual;  so  that  migratory  and  sta- 
tionary half  nuclei  contain  the  same  set  of  chromosomes.  In 
the  one  race  they  will,  of  course,  therefore  both  contain  a 
black  chromosome,  in  the  other  a  white  one.  When  the  ex- 
change and  union  of  half  nuclei  occur,  the  resulting  nucleus 
contains  one  white  chromosome,  one  black  (Figure  51,  F  1). 
This  will  happen  in  every  pair  of  the  two  races  that  mate 
together;  every  family  will  have  a  white-black  pair  of 
chromosomes. 

In  ordinary  fission,  however,  the  black  and  white  do  not 
separate,  but  each  merely  splits,  so  that  all  the  individuals 
produced  have  this  white-black  chromosome  pair  (F  1). 
This  is  also  just  the  situation  of  affairs  in  the  first  genera- 
tion of  offspring  (the  "F  1  generation")  from  a  cross  in 
higher  organisms ;  this  F  1  generation  is  composed  of  indi- 
viduals that  are  alike  with  respect  to  their  characters. 

But  suppose  that  after  a  long  series  of  generations,  the 
white-black  individuals  mate  among  themselves  (as  at  P  2, 
Figure  51 ) .  What  will  happen  ? 

In  all  the  individuals,  one  of  the  two  chromosomes  of  this 
pair  will  be  gotten  rid  of  at  the  second  maturation  division, 
leaving  the  other.  In  half  the  cases  it  will  be  the  black 
chromosome  that  is  left ;  in  half  the  white  one.  That  is,  half 
the  individuals  will  have  black  chromosomes  in  their  migra- 
tory and  stationary  nuclei,  while  half  will  have  white  ones. 

Of  those  that  contain  black  chromosomes,  half  will  mate 
with  other  individuals  that  contain  black  ones,  half  with 
those  that  contain  white  ones.  Similarly,  of  course,  of  those 


Mendelian  Inheritance 
P2 


189 


Figure  51.  Diagram  to  illustrate  how  Mendelian  inheritance  would 
occur  in  an  infusorian  (Paramecium).  The  circles  represent  a  pair  of 
diverse  chromosomes,  the  diversity  being  indicated  by  making  one  black, 
the  other  white.  The  diagram  shows  that  in  successive  conjugations 
these  would  be  distributed  according  to  Mendelian  rules.  After  the 
first  conjugation  (P),  the  ex-con j ugants  and  their  descendants  by 
fission  (Fj)  would  all  have  one  black,  one  white,  chromosome  of  this 
pair.  At  the  next  conjugation  (P2),  by  the  varied  reductions  and 
matings  the  ex-conjugants  and  their  descendants  by  fission  (Fa)  would 
exist  in  the  proportions: — 1  white-white:  2  white-black:  1  black-black. 


190          Life  and  Death,  Heredity  and  Evolution 

that  bear  white  chromosomes,  half  will  have  mates  with  black 
chromosomes,  half  with  white.  Then  all  the  different  com- 
binations so  producible  are  shown  at  P  2,  Figure  51 ;  each 
of  these  combinations  occurs  as  frequently  as  any  other.  By 
two  of  the  four  combinations  we  get  offspring  (  ex-con  ju- 
gants,  F  2)  with  one  chromosome  white,  one  black;  by  one 
we  get  offspring  with  both  chromosomes  white;  by  one,  off- 
spring with  both  chromosomes  black.  Summing  up  the 
eight  offspring,  we  get  the  following  proportions  for  the 
offspring  of  the  generation  F  2: 

2  white-white  -)-  4  white-black  -f-  2  black-black. 

Now  this  is  exactly  what  is  called  Mendelian  inheritance. 
If  we  call  white  A  and  black  a,  the  proportions  give  the  fa- 
miliar Mendelian  formula 

AA  -f  2  Aa  +  aa 

Any  other  chromosome  pair,  or  any  character  that  de- 
pends on  a  chromosome  pair,  will  give  the  same  result. 

We  have  found  then,  so  far  as  knowledge  has  gone  in  this 
direction,  that  mating  produces  the  same  kinds  of  results 
in  the  Protozoa  that  it  does  in  higher  animals ;  it  gives  bi- 
parental  inheritance,  and  also  gives  many  diverse  hereditary 
stocks,  and  these  results  are  produced  in  the  same  way  in 
the  Protozoa  that  they  are  in  higher  organisms. 

The  Effect  of  Mating  on  the  Stock 

What  effect  has  this  on  the  entire  species  in  which  mat- 
ing occurs?  To  answer  this  question,  another  fact  as  to 
conjugation  is  of  importance. 

Assortative  Mating: — When  we  place  together  in  the  same 
vessel  members  of  two  different  races  of  Paramecium,  one 
having  large  individuals,  the  other  small  ones,  and  then  in- 
duce conjugation,  we  observe  a  surprising  fact.  Members 


Assortative  Mating 


191 


of  each  race  mate  only  with  members  of  their  own  race ;  the 
large  individuals  only  with  other  large  ones ;  the  small  in- 
dividuals only  with  other  small  ones  (Figure  52).  There  is 
no  crossing  between  the  two  races  when  they  thus  differ 
considerably  in  size.  This  is  a  highly  inconvenient  fact 
when  one  wishes  to  study  heredity  in  such  crosses ! 


Figure  52.  Conjugants  and  non-con jugants  from  a  culture  composed 
of  a  mixture  of  two  races  (k  and  i)  of  different  size,  of  Paramecium 
aurelia.  The  members  of  the  race  k  are  larger  than  those  of  the  race 
t;  only  members  of  the  same  race  mate  together.  After  Jennings,  1911. 

The  same  thing  is  observed  when  a  culture  of  Paramecium 
contains  individuals  of  many  different  sizes  (whether  of  the 
same  race,  or  of  different  races).  There  is  a  marked  as- 
sortative  mating;  that  is,  individuals  of  the  same  size  tend 
to  mate,  while  individuals  of  diverse  size  do  not  readily  mate 
(Figure  53).  There  are  some  exceptions;  we  find  a  few  in- 
stanees  in  which  a  small  individual  has  mated  with  a  larger 
one,  but  such  cases  are  rare.  In  general,  we  find  that  all 
the  pairs  can  be  arranged  in  a  rather  regular  series  such  as 
Figure  53  shows, — the  two  members  being  of  about  the  same 
size. 

This  assortative  mating  takes  place  with  respect  to  other 


192          Life  and  Death,  Heredity  and  Evolution 


characters  also.  It  has  been  demonstrated  by  careful  study 
that  it  occurs  in  Paramecium  with  reference  to  rate  of  fis- 
sion; the  two  animals  which  mate  have  on  the  whole  similar 
rates  of  fission.  It  appears  clear  that  the  mating  is  between 
animals  of  similar  physiological  characteristics. 

Such  assortative  mating  has  been  shown  to  occur  with 
respect  to  size  in  certain  other  infusoria, — noticeably, 
Blepharisma  (Watters,  1912)  and  Anoplophrya  (Collin, 
1909).  Assortative  mating  is  common,  too,  in  higher  ani- 
mals and  man.  It  is  well  known  how  strong  a  reluctance 


Figure  53.  Pairs  from  a  single  race  of  Paramecium  aurelia,  illus- 
trating assortative  mating;  individuals  alike  in  size  mate  together.  The 
lines  A-C  and  B-BX  are  parallel.  After  Jennings,  1911. 

there  is  in  man  for  strikingly  different  races  to  mate;  a  re- 
luctance that  is  reenforced  by  all  sorts  of  social  and  legal 
regulations  (which  regulations,  of  course,  are  manifestations 
of  the  biological  characteristics  of  the  organisms).  In  the 
case  of  blacks  and  whites  among  human  beings,  for  example, 
an  observer  from  Mars,  examining  in  the  United  States  the 
two  stocks  objectively,  would  find  that  in  the  overwhelming 
majority  of  cases  white  is  mated  with  white,  black  with 
black, — although  some  exceptions  occur. 

In  higher  animals  this  assortative  mating  manifests  itself 
in  details  also,  as  it  does  in  Paramecium;  study  shows,  for 
example,  that  on  the  whole  tall  persons  tend  to  mate  with 
tall,  short  with  short.  Although  detailed  studies  have  been 


General  Results  of  Mating  193 

made  for  but  few  cases,  this  tendency  for  like  to  mate  with 
like,  and  to  refuse  to  mate  with  unlike,  probably  exists  in 
considerable  degree  throughout  the  world  of  organisms. 
This  is  one  of  the  important  facts  to  be  reckoned  with  in 
attempting  to  get  any  general  picture  of  the  results  of 
mating. 

General  Results  of  Mating: — To  a  picture  of  the  general 
results  of  mating  we  now  turn.  In  what  respect  does  the 
world  of  organisms,  or  any  particular  group  of  organisms, 
differ  from  the  condition  which  we  would  find  if  no  mating 
occurred?  We  leave  out  of  account  here  the  results  of  the 
replacement  of  the  old  active  nucleus  by  the  reserve  nucleus, 
since  this  is  not  a  distinctive  feature  of  mating,  occurring  as 
it  does  in  the  infusoria  without  mating;  and  in  most  organ- 
isms not  occurring  even  at  mating. 

We  hear  it  maintained  on  the  one  hand  that  mating  pro- 
duces variation;  some  assert,  indeed,  that  it  is  the  great 
source  of  variation.  On  the  other  hand,  some  maintain  that 
the  result  of  mating  is  to  prevent  or  destroy  variation ;  to 
keep  the  species  of  organisms  uniform.  Facts  can  be  ad- 
duced that  support  both  these  propositions. 

The  difficulty  here  is  that  the  expressions  "increase  varia- 
tion" or  "decrease  variation"  are  ambiguous,  and  that 
neither  of  them  precisely  touches  the  essential  point.  The 
increase  and  decrease  of  variation  are  mere  diverse  aspects 
of  what  really  occurs ;  sometimes  one  of  these  may  result 
from  mating,  sometimes  the  other.  The  really  fundamental 
thing  that  mating  does  is  to  produce  new  combinations  of 
hereditary  characters.  And  in  so  doing  it  quite  changes  the 
face  of  the  world  of  organisms. 

We  may  illustrate  this  most  simply  by  noticing  again  what 
happens  in  the  case  of  the  primary  hereditary  characters, — 
the  chromosomal  packets ;  we  know  that  the  secondary  char- 


194«          Life  and  Death,  Heredity  and  Evolution 

acters  follow  the  primary  ones.  If  there  are  two  races  of 
organisms,  in  each  of  which  both  members  of  each  pair  of 
chromosomes  are  alike  (so-called  pure  lines,  or  pure  homozy- 
gotes),  we  may  call  their  two  sets  of  chromosomes 

ABCD  abed 

ABCD        and       abed 

Now,  when  the  two  sets  are  reduced  in  number  by  division 
into  two  groups,  with  one  member  of  each  pair  in  each  group, 
evidently  the  only  possible  groups  are  ABCD  from  one 
race,  and  abed  from  the  other ;  all  the  half  nuclei  of  each 
race  will  be  alike.  When  two  half  nuclei  from  the  different 
races  mate,  all  the  resulting  nuclei  will  show  the  combination 

ABCD 
abed 

That  is,  by  the  crossing  of  these  two  diverse  races,  progeny 
are  produced  that  are  all  alike.  A  new  combination  has 
been  produced, — but  only  one  combination  from  the  original 
two.  So  the  progeny  of  the  cross  will  be  uniform,  while  the 
parents  were  diverse.  Variation  has  been  greatly  decreased. 

But  now  suppose  that  the  progeny,  showing  this,  new  and 
uniform  combination,  mate  among  themselves.  We  have  al- 
ready for  another  purpose  examined  the  results  of  this 
(page  186)  ;  we  found  that  a  great  number  of  different 
hereditary  combinations  would  be  produced,  such  as 

AbcD    ,    abCd    ,    etc.,  etc. 
AbCd        aBCD 

Progeny  of  81  different  kinds  of  hereditary  combinations 
will  result.  By  this  mating  of  two  parents  that  were  just 
alike  variation  has  been  greatly  increased. 

Suppose  that  we  compare  this  group  of  81  diverse  com- 
binations with  the  two  sets  of  grandparents.  Has  variation 


Effect  of  Mating  on  Variability  195 

been  increased  or  decreased?  In  the  original  stocks  the  two 
sets  of  parents  were  diverse  in  all  their  characters,  while  in 
their  grandchildren,  although  there  are  81  sets  instead  of 
two,  they  show  all  possible  intermixtures  and  gradations. 
It  may  be  maintained  therefore  that  the  result  of  mating  has 
been  to  reduce  the  degree  of  diversity.  And  if  we  find  the 
"coefficient  of  variation"  for  the  two  sets,  which  is  a  measure 
of  how  much  on  the  whole  the  individuals  differ  from  the 
average  intermediate  condition,  we  shall  doubtless  find  this 
to  be  much  greater  for  the  two  original  stocks,  where  none 
of  the  individuals  are  like  the  average  of  the  two  sets,  than 
for  their  grandchildren.  By  this  measure,  therefore,  varia- 
tion will  be  found  reduced  by  mating.  Walton  (1915)  has 
shown  that  precisely  this  is  what  occurs  when  diverse  stocks 
of  certain  higher  organisms  are  mated ;  from  which  he  argues 
that  mating  decreases  variation.  The  whole  is  an  excellent 
illustration  of  the  way  in  which  average  measures  (like  the 
coefficient  of  variation)  may  conceal  important  biological 
facts.  What  has  happened  is  the  production  of  many  di- 
verse hereditary  combinations  bridging  the  gap  between  the 
two  that  first  existed.  This  is  an  increase  of  variation,  if 
one  means  thereby  an  increase  in  the  number  of  hereditarily 
diverse  stocks ;  it  is  a  decrease  of  variation,  if  one  means  by 
,ariation  the  average  diversity  from  the  intermediate  con- 
dition. But  this  superficial  increase  or  decrease  of  variation 
is  merely  a  consequence  of  the  underlying  fact, — the  pro- 
ductions of  new  combinations  through  mating.4 

4  Mast  (1917)  studied  this  phenomenon  of  increase  or  decrease  in 
variability  of  the  fission  rate  as  a  result  of  mating  in  the  infusorian 
Didinium.  He  was  not  able  to  detect  with  certainty  any  consistent 
increase  or  decrease  in  the  coefficient  of  variation  as  a  result  of  mating. 
This  need  not  surprise  us,  in  view  of  the  points  brought  out  above;  but 
Mast's  data  were  in  any  case  hardly  such  as  to  give  dependable  results 
on  the  matter.  The  study  of  variation  was  made  on  the  results  of 
experiments  designed  for  entirely  different  purposes,  so  that  the  num- 
bers of  cases  were  too  small  to  give  significant  data.  Most  of  his  coeffi- 


196         Life  and  Death,  Heredity  and  Evolution 

The  facts  of  assortative  mating,  of  course,  limit  the  ex- 
tent to  which  new  combinations  of  the  characters  are  pro- 
ducible through  mating.  After  stocks  have  reached  a  cer- 
tain degree  of  diversity,  assortative  mating  prevents  their 
union,  and  so  prevents  the  formation  of  stocks  combining 
their  characteristics.  But  this  still  leaves  a  wide  field  for 
the  formation  through  mating  of  varied  combinations  of  the 
hereditary  characteristics  of  differing  stocks. 

We  now  come  to  one  of  the  most  striking  biological  re- 
sults of  mating.  In  this  formation  of  new  combinations, 
characters  which  were  previously  in  diverse  stocks  become 
united  in.  one  individual.  Sometimes  the  characters  so  com- 
bined are  more  or  less  incompatible;  they  do  not  develop 
well  together ;  or  they  do  not  function  harmoniously ;  or  they 
produce  secondary  characters  which  do  not  function  well 
under  the  particular  conditions  in  which  the  organisms  are 
found.  Individuals  resulting  from  such  combinations  may 
be  weak  or  pathological;  they  may  not  develop  at  all;  or 
if  they  do,  they  lack  vigor,  and  do  not  multiply.  Many  ex- 
amples of  such  consequences  we  saw  in  our  study  of  the  re- 
sults of  mating  in  Paramecium;  many  individuals  or 
families  produced  died  out,  or  were  weak,  multiplying 
slowly;  or  showed  hereditary  deformities.  Other  individu- 
als on  the  other  hand  received  combinations  of  characters 

cients  of  variation  were  worked  out  on  the  basis  of  but  2,  3  or  4  diverse 
lines  of  descent!  In  only  one  single  case  were  there  as  many  as  eight 
lines  in  both  the  sets  compared  (conjugants  and  non-con jugants);  in 
this  case  the  variation  was  much  greater  in  the  lines  that  had  conju- 
gated. But  it  is  obvious  that  such  small  numbers  cannot  give  clear  and 
consistent  results,  particularly  when  we  find  that  the  coefficients  of 
variation  brought  to  light  range  from  0  to  55.3  per  cent.  If  probable 
errors  had  been  worked  out,  they  would  doubtless  have  shown  the  com- 
parisons of  coefficients  to  be  without  significance.  A  study  of  the  ques- 
tion whether  mating  actually  produces  lines  with  diverse  hereditary 
characters  would  be  of  great  interest  in  Didinium,  as  in  any  other  Pro- 
tozoa, but  the  determination  of  coefficients  of  variation  is  a  most  imper- 
fect and  uncertain  index  of  this  matter. 


Mating  and  Selection  197 

that  worked  well  together  and  under  the  conditions  of  the 
environment;  the  families  produced  were  vigorous,  some  of 
them  flourishing  even  better  than  the  original  stocks  through 
whose  mating  they  were  produced.  In  the  same  manner  a 
poor  family  of  human  beings  may  produce  a  Lincoln  among 
its  children;  a  combination  of  characteristics  is  formed  by 
mating  that  never  before  existed  and  that  meets  the  condi- 
tions of  existence  in  a  more  vigorous  and  successful  manner 
than  the  individuals  from  which  it  was  derived. 

Experimentally,  or  under  natural  conditions,  of  course 
these  newly  formed  stocks  in  which  life  and  reproduction  are 
vigorous,  gradually  replace  the  weakened  stocks,  in  which 
mating  has  resulted,  not  in  reinvigoration  but  in  degenera- 
tion ;  in  the  formation  of  combinations  of  primary  hereditary 
characters  that  cannot  develop  vigorously  under  the  condi- 
tions. The  formation  of  such  "degenerated"  stocks  is  as 
much  a  characteristic  of  mating  as  the  formation  of  more 
vigorous  ones. 

By  this  formation  of  new  combinations  with  gradual  re- 
placement of  the  unenduring  ones  by  those  £hat  are  vigor- 
ous, the  stocks  in  existence  come  to  be  very  diverse  from 
those  that  existed  at  an  earlier  time.  Mating  is  a  continued 
process  of  forming  new  combinations  of  the  primary  heredi- 
tary characters ;  of  the  chemicals  on  which  the  vigor  and 
the  nature  of  development  depend ;  with  suppression  of  the 
combinations  that  are  weak  or  imperfect,  leaving  the  more 
harmonious  and  vigorous  combinations  in  existence,  to  carry 
the  process  further. 


VIII 


Comparison  of  the  Genetic  Phenomena  in  the  Protozoa 
with  Those  in  Higher  Organisms.  General  View  of  Develop- 
ment, Mating  and  Evolution. 

HOW  far  does  the  condition  of  affairs  which  we  have  set 
forth  for  the  lower  organisms  hold  for  the  higher 
ones  ? 

As  we  have  remarked  at  various  points  in  our  earlier 
lectures,  many,  perhaps  most,  of  the  general  relations  are 
similar  in  the  lower  and  higher  organisms.  But  there  are 
certain  points  in  regard  to  which  questions  may  be  raised; 
particularly  as  to  the  origin  of  new  hereditary  characters. 
The  points  needing  examination  are  mainly  (1)  as  to  the 
effect  of  the  environment  in  causing  inherited  changes,  and 
(2)  as  to  the  nature  and  extent  of  the  changes  in  hereditary 
characters  that  arise  in  nature;  with  the  relation  of  these 
to  the  process  of  evolution. 

We  have  dealt  in  our  fourth  lecture  with  alteration  of  the 
hereditary  constitution  of  lower  organisms  by  external  con- 
ditions. In  the  higher,  more  complex  organisms  the  under- 
lying conditions  as  to  this  are  somewhat  different  from  those 
found  in  organisms  composed  of  but  a  single  cell;  though 
perhaps  not  so  completely  diverse  as  is  sometimes  conceived. 
But  in  the  higher  organisms  there  is  a  great  mass  of  cells, 
the  body,  which  finally  disintegrates  completely,  without 
propagating  itself  by  division ;  the  body  of  the  next  genera- 
tion is  formed,  not  from  the  body  of  the  preceding  genera- 
198 


Inheritance  of  Acquired  Characters  199 

tion,  but  by  the  divisions  of  a  single  cell,  which  is  formed  by 
the  union  of  two  half  cells,  one  from  each  parent.  This  was 
illustrated  in  Figure  5,  on  page  21. 

It  is  clear  that  if  any  of  the  later  (secondary)  hereditary 
characters  are  to  become  modified,  this  must  be  accomplished 
by  some  modification  of  the  primary  hereditary  characters, 
— those  passed  on  bodily  from  parent  to  offspring.  For 
if  two  germ  cells  are  exactly  alike,  the  characters  inherited 
through  them  are  bound  to  be  alike;  two  individuals  that 
are  to  differ  in  their  later  hereditary  characters  must  be 
diverse  in  their  primary  hereditary  characters.  If  environ- 
mental agencies  are  to  produce  diversities  that  are  to  be 
hereditary,  they  must  change  the  germ  cells,  in  the  particular 
way  required  to  bring  about  the  observed  later  changes  in 
the  body ;  for  the  body  is  derived  from  these  germ  cells. 

This  has  always  been  the  theoretical  difficulty  with  the 
"inheritance  of  acquired  characters," — if  we  mean  by  that 
abused  expression  the  inheritance  of  modifications  produced 
directly  on  the  body  by  the  outer  world.  If  the  form  of  the 
hand  is  changed  by  certain  outward  conditions,  how  is  that 
change  to  modify  the  primary  hereditary  characters  in  the 
germ  cells,  which  are  not  directly  touched  by  the  given 
outer  conditions, — in  just  such  a  way  as  to  cause  them  to 
produce  the  same  new  form  of  hand  in  the  next  generation? 

Theoretical  difficulties  of  this  sort  of  course  must  not 
be  allowed  to  stand  in  the  way  of  our  recognizing  how  nature 
actually  does  operate,  if  she  does  indeed  operate  without 
regard  to  these  difficulties.  There  is  nothing  so  little  worthy 
of  confidence  in  science  as  assertions  that  particular  events, 
not  yet  observed,  are  impossible ;  such  propositions  have  been 
falsified  a  thousand  times,  and  the  careful  man  of  science, 
will  not  permit  his  researches  to  be  guided  by  them.  But  in 
this  case  the  great  weight  of  evidence  thus  far  is  that  this 


200         Life  and  Death,  Heredity  and  Evolution 

particular  theoretical  difficulty  corresponds  to  an  actual 
one;  that  the  direct  effect  of  the  environment  on  the  bod}' 
cells  is  not  inherited  through  the  germ  cells  in  the  next 
generation.  It  is  only  fair  to  say  however  that  there  is 
certain  evidence  produced  by  the  Austrian  investigator 
Kammerer  (1913)  in  long-continued  experimental  studies  on 
amphibians  which  seems  to  imply  such  inheritance  through 
the  germ  cells  of  changes  primarily  produced  in  the  body 
cells  of  the  animal.  But  practically  all  students  of  biology 
will  agree  that  this  evidence  is  far  from  establishing  heredity 
of  this  sort,  and  that  the  overwhelming  mass  of  evidence  is 
against  it. 

But  it  is  another  question  whether  external  agents  may 
not  act  directly  on  the  germ  cells,  in  such  a  way  as  to  induce 
them  to  produce  a  body  with  new  characteristics,  and  to 
transmit  the  same  changes  by  cell  division  to  the  germ  cells 
that  are  to  produce  the  later  generations,  so  that  these  too 
show  the  altered  hereditary  characters.  This  sort  of  action 
would  correspond  to  the  hereditary  changes  produced  by 
external  agents  in  the  Protozoa  and  bacteria,  such  as  we 
described  in  Lecture  4. 

There  is  no  theoretical  difficulty  whatever  as  to  this ;  the 
difficulties  are  purely  observational  ones;  it  turns  out  that 
such  changes  do  not  occur  so  readily  or  frequently  as  one 
would  expect.  There  is  no  a  priori  reason  why  the  sub- 
stances of  the  germ  cells  should  not  be  as  readily  altered 
as  any  other  mass  of  chemicals.  But  in  practice  it  turns 
out  that  most  agents  which  produce  chemical  alterations 
of  the  germ  cells  at  the  same  time  kill  the  organism.  Further, 
the  germ  cells,  like  other  living  systems,  have  a  great 
tendency  to  compensate  for  any  disturbances  induced  them; 
their  condition  is  one  of  stable  equilibrium,  in  which  an  alter- 
ation is  followed  by  a  return  to  the  original  condition.  Add 


Production  of  Inherited  Variations  201 

to  this  the  fact  that  in  higher  organisms  the  germinal  mate- 
rial is  commonly  hidden  deeply  within  a  great  mass  of  body 
cells,  by  which  its  surroundings  are  kept  uniform  and  it  is 
protected  from  marked  changes  of  all  sorts, — and  it  becomes 
more  intelligible  why  in  higher  organisms  inherited  changes 
due  to  the  action  of  the  environment  are  much  less  commonly 
observed  than  general  theory  might  lead  us  to  expect.  In 
the  bacteria  and  Protozoa  the  germinal  material  is  not  pro- 
tected by  a  great  mass  of  body  cells,  but  is  more  directly 
exposed  to  the  action  of  environmental  agents,  so  that  in 
these,  heritable  results  of  environmental  action  are  better 
known. 

When  we  examine  the  experimental  evidence  on  this  matter 
in  higher  organisms,  we  find  that  scientific  opinion  looks 
upon  it  as  being  in  a  somewhat  less  satisfactory  condition 
than  appeared  to  be  the  case  a  few  years  ago.  Cases  had 
been  described  in  which  the  inheritance  of  the  action  of  the 
environment  on  the  germ  cells  appeared  clear;  and  the  evi- 
dence on  these  particular  cases  has  not  altered.  But  the 
lack  of  further  confirmation,  of  other  instances ;  the  failure 
of  other  tests  under  similar  conditions,  has  shaken  the 
opinion  of  most  students  of  biology  as  to  the  conclusiveness 
of  the  evidence  that  had  been  given,  and  has  made  them 
inclined  to  wait  for  further  evidence  before  accepting  the 
principles  of  action  involved.  No  subject  in  biology  is  more 
in  need  of  further  work  than  this  one. 

The  principal  evidence  for  actual  modification  of  the 
germ  cells  by  the  environment  in  such  a  way  as  to  cause  the 
appearance  in  the  body  of  new  hereditary  characters  has 
come  perhaps  from  the  work  of  Standfuss  (1906)  and 
Fischer  (1907);  of  Tower  (1906);  of  Kammerer  (1913), 
and  of  Stockard  (1913);  these  at  least  are  typical.  It  is 
not  our  present  task  to  give  an  account  of  these  researches, 


20£          Life  and  Death,  Heredity  and  Evolution 

but  a  few  words  of  comment  will  aid  in  obtaining  an  outline 
of  the  present  situation  in  science. 

Standfuss  and  Fischer,  by  the  application  of  low  tem- 
peratures to  the  pupae  of  certain  butterflies,  induced  in  a 
small  proportion  of  the  adults  the  appearance  of  darker 
tinges  of  color  than  usual.  In  a  small  proportion  of  these 
aberrant  adults,  the  modified  color  reappeared  in  later  gen- 
erations; it  was  hereditary. 

Tower  tried  the  effects  of  hot,  moist  conditions  on  the 
potato  beetle  at  the  time  that  the  germ  cells  were  under- 
going their  growth  and  transformation.  He  found  that 
the  germ  cells  so  treated  produced  a  considerable  proportion 
of  individuals  differing  from  the  typical  ones ;  lighter  in- 
dividuals and  darker  ones.  And  in  later  generations  these 
aberrant  colors  showed  themselves  to  be  hereditary. 

Kammerer  experimented  for  many  years  in  breeding  vari- 
ous sorts  of  amphibians,  attempting  through  climatic 
changes,  alterations  of  temperature  and  moisture,  changes  in 
color  of  the  background  on  which  they  live,  and  'by  other 
means,  including  operations,  to  modify  their  colors,  habits 
and  other  characters.  According  to  his  detailed  reports, 
published  in  the  technical  scientific  journals,  he  has  been  re- 
markably successful  in  this ;  many  sorts  of  diversities  in  en- 
vironmental conditions  have  produced  inherited  alterations ; 
including  even  operative  procedures.  These  positive  results 
have  come  to  him  so  easily  and  regularly,  in  experiments  of  a 
sort  in  which  others  have  practically  universally  reached  neg- 
ative results,  as  to  arouse  in  most  investigators  a  feeling  that 
they  must  be  confirmed  by  others  before  they  can  be  accepted. 
It  must  be  said  however  that  Kammerer's  work  appears  to 
have  been  done  with  great  thoroughness  and  care,  and  he 
has  given  full  account  of  his  methods  and  results,  in  such  a 
way  as  to  leave  little  room  for  criticism  of  details.  On  the 


Production  of  Inherited  Variations  203 

other  hand  the  phenomena  with  which  he  deals  are  excessively 
complicated  and  variable,  making  errors  of  interpretation 
easy  to  one  who  is  strongly  convinced  of  a  particular 
doctrine,  as  Kammerer  evidently  is  of  the  inheritance  of  en- 
vironmental effects.  Some  results  and  interpretations  re- 
ported by  him  have  put  a  strong  strain  on  the  powers  of 
acceptance  of  other  investigators ;  notably  his  report  of 
the  inherited  effect  of  cutting  off  a  certain  organ  in  the 
ascidian ;  1  and  his  report  that  when  the  ovary  of  a  given 
sort  of  salamander  is  transplanted  to  the  body  of  another 
kind,  the  germ  cells  of  this  ovary  transmit  the  characters  of 
the  body  to  which  they  were  transplanted.  Kammerer's 
work  is  distinctly  in  need  of  confirmation. 

Stockard  dealt  with  the  effects  of  alcohol  on  the  germ 
cells,  and  through  these  on  the  later  generations  of  off- 
spring in  the  Guinea  pig.  He  found  that  continued  ad- 
ministration of  alcohol  to  the  parents  so  injures  their  germ 
cells  that  their  progeny  and  their  descendants  of  later  gen- 
erations are  weak,  imperfect,  diseased,  deformed,  in  many 
ways. 

Somewhat  similar  results  had  been  reached  years  before  by 
Brown-Sequard  (1869),  in  studying  the  results  of  mutilation 
of  the  parent  in  Guinea  pigs.  Repetition  of  his  work  by 
later  investigators  has  not  convinced  students  of  the  subject 
that  it  was  correctly  interpreted;  it  is  believed  that  he  was 
dealing  with  diseased  stocks.  The  study  made  by  Pearl 
(1917  a)  of  the  effects  of  alcohol  in  the  fowl  did  not  bring 
to  light  any  such  results  as  Stockard's  on  the  Guinea  pig, 
so  that  there  is  on  the  whole  a  tendency  to  suspend  judg- 
ment as  to  the  interpretation  of  the  results  until  Stockard's 
work  has  been  repeated  on  a  new  stock. 

Thus  all  along  the  line  there  is  a  feeling  of  uncertainty 
1  See  the  comment  of  Castle^  1916o,  page  29,  on  this  point. 


204          Life  and  Death,  Heredity  and  Evolution 

and  a  desire  for  further  tests  before  judgment  is  passed  as  to 
the  inherited  effects  of  environmental  action  on  the  germ 
cells,  in  higher  organisms ;  more  work  along  these  lines  is 
greatly  needed. 

But  the  results  set  forth  are,  in  all  cases  save  perhaps 
certain  of  those  described  by  Kammerer,  of  the  sort  that 
agree  in  principle  with  the  inherited  effects  of  environmental 
action  in  the  lower  organisms,  as  described  in  Lecture  4. 
The  action  of  the  external  agents  was  directly  on  the  germ 
cells,  modifying  the  primary  (directly  transmitted)  heredi- 
tary characters.  In  consequence  the  later  or  secondary 
characters  were  altered.  Such  cases  are  perhaps  better 
established  in  the  Protozoa  and  bacteria  than  in  higher 
organisms. 

As  to  the  nature  and  extent  of  the  changes  in  hereditary 
characters  arising  in  higher  organisms,  aside  from  the  direct 
effects  of  environmental  action,  an  enormous  volume  of  work 
has  been  done.  In  higher  organisms  it  is  a  prevalent  con- 
viction that  changes  in  the  hereditary  characters  occur  by 
mutation.  What  relation  has  this  and  the  facts  on  which 
it  is  based  to  the  hereditary  changes  seen  in  the  Protozoa, — 
in  such  a  case  as  Difflugia,  for  example? 

The  concept  of  mutation  has  been  based,  in  different 
minds,  on  a  number  of  different  points, — sometimes  held 
separately,  sometimes  together.  One  basis  for  the  distinc- 
tion of  mutations  from  other  variations  is  this :  Many 
variations  in  the  characteristics  of  organisms  are  not  in- 
herited ;  such  are  the  common  superficial  effects  of  environ- 
mental diversities.  We  have  given  illustrations  of  this  in 
our  account  of  the  Protozoa.  It  is  therefore  convenient  to 
have  a  distinctive  name  for  those  that  are  inherited,  and 
some  call  these  mutations.  In  this  usage,  of  course  it  is  a 
mere  matter  of  definition  to  state  that  any  new  heritable 


Mutation  205 

variation  is  a  mutation.  The  word  so  used  implies  nothing 
as  to  extent  or  nature  of  the  variations  that  are  herit- 
able;  it  is  a  name  for  all  that  occur. 

But  in  many  minds  the  term  mutation  means  more  than 
this.  Many  new  heritable  characters  in  higher  organisms 
are  found  to  be,  when  the  character  has  reached  its  com- 
plete development  in  the  adult,  changes  of  considerable  ex- 
tent. They  differ  from  the  original  condition  by  a  large 
step ;  they  affect  many  parts  of  the  organism ;  or  profoundly 
change  particular  organs.  Such  were  most  of  the  hereditary 
changes  found  by  de  Vries  (1901)  in  the  evening  primroses, 
CEnothera ;  from  this  work  arises  the  general  use  of  the  word 
mutation  for  hereditary  changes.  Such  too  are  many  of  the 
hereditary  changes  observed  in  the  fruit-fly,  Drosophila,  by 
Morgan  and  his  associates.  Thus,  in  the  typical  individuals 
the  eyes  are  red ;  these  sometimes  produce  offspring  in  which 
the  eyes  are  white,  and  this  mutation  is  inherited.  The 
hereditary  change  has  come,  not  by  minute  changes  of  shade, 
gradually  altering  from  generation  to  generation;  but  by  a 
complete  change  in  one  generation  from  red  to  white. 

From  such  cases,  the  word  mutation  has  come  to  mean  in 
the  minds  of  many  persons  an  extensive  change;  a  sudden 
jump  from  one  condition  to  another;  a  "saltation."  And 
the  statement  that  evolutionary  changes  occur  by  mutation 
has  come  to  mean  that  they  do  not  take  place  in  gradations ; 
in  minute,  almost  imperceptible  alterations  from  generation 
to  generation,  but  always  by  large  leaps.  Possibly  this  is 
the  usual  idea  of  what  is  meant  by  the  mutation  theory  of 
evolution. 

Such  slight  changes  as  we  have  described  in  the  preceding 
lectures  as  occurring  in  Difflugia  and  other  Protozoa  do  not 
agree  with  this  idea.  Is  there  a  contrast  in  this  respect 
between  what  occurs  in  the  higher  and  the  lower  organisms? 


206         Life  and  Death,  Heredity  and  Evolution 

It  is  natural  that  an  alteration  of  a  primary  hereditary 
character  in  the  germ  cell  of  a  higher  organism  should,  when 
the  long  development  from  that  germ  cell  is  completed,  pro- 
duce a  much  more  extensive  and  more  marked  effect  than  in 
a  Protozoan.  For  in  the  latter  it  is  the  same  cell  that  is 
altered  which  forms  the  adult,  with  relatively  little  develop- 
ment, and  with  no  intervening  multiplication  of  cells.  But 
in  the  higher  organism  the  altered  germ  cell  goes  through  a 
great  number  of  cell  divisions,  accompanied  by  continuous 
interactions  of  the  different  substances  in  the  nuclei,  result- 
ing in  an  enormous  increase  in  differentiation,  in  numbers  of 
cells,  and  in  bulk.  All  these  cells,  and  this  entire  bulk,  may 
therefore  show  the  results  of  the  slight  original  change.  If 
some  substance  necessary  for  the  production  of  the  red  eye 
color  of  the  fruit-fly  were  omitted  from  the  germ  cell,  it  is 
probable  that  the  change  in  the  germ  cell  would  itself  be 
so  slight  that  it  could  not  be  detected  by  any  physical  or 
chemical  tests  at  present  available.  It  is  little  more  than 
changes  corresponding  to  this  that  we  may  expect  to  find 
in  the  organisms  made  up  of  but  one  cell. 

But  must  a  hereditary  change  in  the  adult  characters  of 
a  higher  organism  necessarily  be  such  a  saltation ;  a  change 
of  large  extent?  On  this  point  the  state  of  knowledge  has 
greatly  changed  with  the  thorough  studies  made  in  recent 
years,  although  the  change  has  as  yet  been  little  appreciated 
outside  the  field  of  specialists  working  on  these  matters. 
We  shall  attempt  to  give  a  brief  sketch  of  the  position  of 
this  question  in  higher  organisms,  for  comparison  with  what 
we  have  seen  in  the  lower  ones. 

It  has  been  found  that  in  many  higher  organisms  it  is 
possible  through  long  continued  breeding  with  careful  selec- 
tion of  the  parents,  to  gradually  cause  a  change  in  the  hered- 
itary characteristics  shown  by  the  stock.  Often  this  change 


Nature  of  Inherited  Variations  207 

is  a  mere  quantitative  alteration,  in  the  extent  or  intensity  of 
pigmentation  or  other  characteristics.  In  many  cases  such 
alterations  have  occurred  in  characters  that  in  other  re- 
spects behave  like  "single  unit  characters."  Such  work  was 
done  on  the  rat  by  Castle  and  his  associates;  on  Drosophila 
by  MacDowell,  Zeleny  and  Mattoon,  Reeves,  Morgan,  Stur- 
tevant,  and  others. 

Two  views  have  been  held  by  investigators  as  to  the  nature 
of  the  change  in  such  cases.  Castle  and  a  number  of  others 
have  long  held  that  there  was  occurring  a  gradual- change, 
perhaps  merely  quantitative  in  nature,  in  the  single  unit 
factor  on  which  the  adult  character  depends.  On  the  other 
side,  many  have  maintained  that  these  gradual  alterations 
are  due  to  the  fact  that  the  adult  character  depends  on 
many  distinct  genes  or  unit  factors,  each  affecting  the  adult 
character  but  little.  By  selective  breeding  many  of  these 
factors  are  gradually  accumulated  in  one  set  of  progeny, 
few  in  the  other;  so  that  the  adult  features  become  slowly 
very  diverse.  That  is,  it  is  maintained  that  the  apparent 
changes  in  the  hereditary  characters  are  really  due,  like  all 
Mendelian  inheritance,  to  recombinations  of  the  existing  fac- 
tors. 

This  explanation,  commonly  called  the  hypothesis  of  mul- 
tiple modifying  factors,  has  recently  been  accepted,  on  the 
basis  of  crucial  experiments,  by  Castle  himself.2  There  can 
hardly  be  doubt  that  it  is  correct  for  at  least  most  cases  of 
this  kind. 

Let  us,  therefore,  accept  this  explanation,  and  proceed  to 
an  examination  of  its  relation  to  the  questions  in  which  we 
are  interested.  What  bearing  have  the  facts,  so  interpreted, 
on  the  nature  of  hereditary  variations  and  on  the  method  of 
evolution  ? 

'Castle,  W.  E.,  Proc.  Nat.  Acad.,  April,  1919. 


208          Life  and  Death,  Heredity  and  Evolution 

In  no  other  organism  have  heritable  variations  been  stud- 
ied so  thoroughly  as  in  Drosophila,  and  no  other  body  of 
men  have  been  more  thoroughgoing  upholders  of  mutation- 
ism  and  of  the  multiple  factor  explanation  of  the  effects  of 
selection,  than  the  students  of  Drosophila — Morgan,  Sturte- 
vant,  Bridges,  Dexter,  Muller,  MacDowell,  and  the  others. 
We  may  therefore  turn  to  the  evidence  from  Drosophila 
with  confidence  that  it  will  be  presented  with  fairness  to  the 
mutationist  point  of  view.  We  shall  first  ask  (\)  what  we 
learn  from  the  work  on  Drosophila  as  to  the  possibility  of 
finding  finely  graded  variations  in  a. single  unit  character. 
Next  we  shall  inquire  (2)  as  to  the  relation  of  the  assumed 
modifying  factors  to  changes  in  hereditary  constitution;  to 
the  nature  of  the  effects  of  selection. 

1.  First,  then,  what  are  the  facts  as  to  numerous  finely 
graded  variations  in  a  single  unit  factor?  Here  we  have 
certain  remarkable  data  as  to  the  eye-color  of  Drosophila; 
data  that  are  of  great  interest  with  relation  to  the  nature 
of  evolutionary  change.  This  fruit-fly  has  normally  a  red 
eye.  Some  years  ago  a  variation  occurred  by  which  the 
eye  lost  its  color,  becoming  white,  a  typical  mutation. 
Somewhat  later  another  variation  came,  by  which  the  eye 
color  became  eosin.  By  these  wonderfully  ingenious  meth- 
ods which  the  advanced  state  of  knowledge  of  the  genetics 
of  Drosophila  has  made  possible,  it  was  determined  that  the 
mutations  white  and  eosin  are  due  to  changes  in  a  particular 
part  of  a  particular  chromosome,  namely,  of  the  so-called 
X-chromosome,  or  chromosome  I.  And  further,  it  was  dis- 
covered that  the  two  colors  are  due  to  different  conditions 
of  the  same  locus  of  the  chromosome ;  in  other  words,  they 
represent  two  different  variations  of  the  same  unit.  More- 
over, the  normal  red  color  represents  a  third  condition  of 
that  same  unit. 

Somewhat  later  a  fourth  condition  of  this  same  unit  was 


Inheritance  of  Small  Variations  209 

found,  giving  a  color  which  lies  nearer  the  red,  between  the 
red  and  eosin ;  this  new  color  was  called  cherry.  So  we  have 
four  grades  or  conditions  of  this  single  unit  character. 

And  now,  with  the  minute  attention  paid  to  the  distinction 
of  these  grades  of  eye  color,  new  grades  began  to  come  fast. 
In  the  number  of  Genetics  for  November,  1916,  Hyde  adds 
two  new  grades,  one  called  "blood,"  near  the  extreme  red  end 
of  the  series,  the  other  called  "tinged,"  near  the  extreme 
white  end;  in  fact,  from  the  descriptions  it  requires  careful 
examination  to  distinguish  these  two  from  red  and  white,  re- 
spectively. Thus  we  have  now  six  grades  of  this  unit.  And  in 
the  same  number  of  the  same  journal,  Safir  (1916)  adds  an- 
other intermediate  grade,  lying  between  "tinged"  and  eosin ; 
this  he  calls  "buff."  All  these  seven  grades  are  diverse  con- 
ditions of  the  single  unit  factor,  having  its  locus  in  a  certain 
definite  spot  in  the  X-chromosome.  Such  diverse  conditions 
of  a  single  factor  are  known  as  multiple  allelomorphs. 

So,  up  to  date  we  know  from  the  mutationists'  own  stud- 
ies of  Drosophila  that  a  single  unit  factor  presents  seven 
gradations  of  color  between  white  and  red,  each  gradation 
heritable  in  the  usual  Mendelian  manner.  These  grades  are 
the  following:  (1)  red;  (2)  blood;  (3)  cherry;  (4)  eosin; 
(5)  buff;  (6)  tinged;  (7)  white. 

It  would  not  require  a  bold  prophet  to  predict  that  as  the 
years  pass  we  shall  come  to  know  more  of  these  gradations, 
till  all  detectible  differences  of  shade  have  been  distinguished, 
and  each  shown  to  be  inherited  as  a  Mendelian  unit.  Con- 
sidering that  the  work  on  Drosophila  has  been  going  on  only 
about  seven  or  eight  years,  this  is  remarkable  progress 
toward  a  demonstration  that  a  single  unit  factor  can  pre- 
sent as  many  grades  as  can  be  distinguished,  that  the  grades 
may  give  a  pragmatically  continuous  series. 

Besides  showing  that  a  unit  factor  may  thus  exist  in 
numerous  minutely  differing  grades,  this  case  shows  that  a 


210          Life  and  Death,  Heredity  and  Evolution 

heritable  variation  may  occur  so  small  as  to  be  barely  de- 
tectible.  Although  the  variations  do  not  usually  occur  in 
this  way,  the  case  present  the  conditions  which  would  allow 
of  a  gradual  transition  from  one  extreme  to  the'other,  by 
means  of  numerous  intermediate  conditions. 

2.  But,  as  we  have  seen,  the  gradual  changes  in  heredi- 
tary characters  seen  in  selective  breeding  usually  do  not  oc- 
cur in  this  way,  but  rather  by  the  slow  accumulation  of 
many  factors  each  having  a  slight  effect, — the  multiple  modi- 
fying factors.  But  what  sort  of  things  are  these  factors  and 
what  is  their  relation  to  actual  changes  in  the  heritable  con- 
stitution of  the  organism? 

Our  direct  experimental  knowledge  of  these  "modifying 
factors"  is  scanty;  it  comes  mainly  from  the  studies  of 
Drosophila.  We  find  data  as  to  certain  known  modifying 
factors  by  Bridges  (1916)  in  his  important  paper  on  non- 
disjunction  of  the  chromosomes.  And  here  we  are  taken 
back  again  to  the  series  of  eye  colors,  and  indeed  to  one  par- 
ticular member  of  the  series,  the  middle  member,  called  eosin. 
Bridges  tells  us  that  he  found  a  factor  whose  only  effect  was 
to  lighten  the  eosin  color  in  a  fly  with  eosin  eyes ;  this  factor 
indeed  nearly  or  quite  turns  the  eosin  eye  white.  This  factor 
Bridges  calls  "whiting."  Another  factor  has  the  effect  of 
lightening  the  eosin  color  a  little  less,  giving  a  sort  of  cream 
color;  this  is  called  "cream  b."  A  third  factor  dilutes  the 
eosin  color  not  so  much ;  it  is  called  "cream  a."  In  addition 
to  these,  Bridges  tells  us  that  he  has  discovered  three  other 
diluters  of  the  eosin  color ;  we  will  call  them  the  fourth,  fifth, 
and  sixth  diluters.  And  finally  Bridges  tells  us  of  another 
factor  whose  only  effect  is  to  modify  eosin  in  the  direction  of 
a  darker  color ;  this  factor  he  calls  "dark."  None  of  these 
factors  has  any  effect  save  on  eosin-eyed  flies. 

As  you  see,  these  things  add  tremendously  to  our  grada- 


Inheritance  of  Small  Variations 


211 


tions  in  eye  color.  We  had  already  been  furnished  seven 
grades,  from  white  to  red;  now  we  have  seven  secondary 
grades  within  a  single  one  of  these  seven  primary  grades. 
Our  list  of  gradations  of  eye  color  in  Drosophila  therefore 
takes  now  the  following  form: 


Heritable  grades  of  eye  color, 
due  to  diverse  variations  of  a 
single  unit  located  in  Chromo- 
some I. 

1.  White 

2.  Tinged 

3.  Buff 

4.  Eosin 

5.  Cherry 

6.  Blood 

7.  Red 


Variations  that  give  modifica- 
tions of  the  intensity  of  eosin,  but 
are  located  in  other  chromosomes. 


1.  Whiting 

2.  Cream  b 

3.  Cream  a 

4.  Fourth  diluter 

5.  Fifth  diluter 

6.  Sixth  diluter 

7.  Dark 


Here  again  then  we  have  minutely  differing  conditions  of 
a  single  shade  of  color,  brought  about  by  seven  modifying 
factors. 

But  what  are  these  modifying  factors?  And  here  we 
come  to  the  essential  point.  These  modifying  factors  are 
themselves  alterations  in  the  hereditary  constitution.  Bridges 
leaves  no  doubt  upon  this  poinb.  He  lists  and  describes  them 
specifically  as  mutations ;  as  actual  changes  in  the  hereditary 
material. 

What  then  is  the  difference  in  principle  between  such  cases 
and  the  theory  of  gradual  alterations  in  a  single  unit  fac- 
tor? The  difference  is  that  in  the  case  of  the  multiple  modi- 
fying factors  the  minute  changes  occur,  not  all  in  one  factor 
— in  one  locus  of  the  chromosome — but  in  a  number  of  di- 
verse parts  of  the  germinal  material;  this  appears  to  have 
been  clearly  demonstrated.  But  this  is  a  matter  of  detail; 
it  does  not  touch  the  fundamental  question. 

This  fundamental  question  is  as  to  the  occurrence  of  these 
minute  changes  in  the  hereditary  constitution,  and  as  to  the 


212          Life  and  Death,  Heredity  and  Evolution 

possibility  of  getting  therefrom  by  selection  various  grades 
of  a  given  external  characteristic.  In  this,  so  far  as  I  can 
see,  there  is  complete  agreement. 

It  appears  then  that  under  the  recent  careful  studies 
made,  it  can  no  longer  be  maintained  that  hereditary  changes, 
even  in  higher  organisms,  must  be  large  leaps  or  saltations. 
They  may  be  of  this  character,  but  they  may  equally  well  be 
graded  changes  so  slight  as  to  be  hardly  detectible  when 
taken  singly. 

This  appears  to  be  recognized  by  those  who  have  proposed 
and  defended  the  mutation  theory.  De  Vries  (1916)  in  a 
recent  summary  of  the  theory  emphasizes  throughout  exten- 
sive mutations,  and  speaks  repeatedly  of  their  origin  as 
"sudden  and  without  transitional  conditions,"  but  admits 
also  that  t(not  only  very  small,  but  also  much  greater"  dif- 
ferences between  species  arise  all  of  a  sudden  ("mit  einem 
Sprunge")  ;  and  that  most  mutations  affect  only  a  single 
character.  He  sets  forth  further  that  in  a  single  stock  one 
such  mutation  after  another  may  arise,  at  intervals,  until  in 
the  course  of  time  the  stock  has  become  very  diverse  from 
the  original  one;  and  has  become  differentiated  into  a  num- 
ber of  different  types  on  which  selection  may  act.  Now,  so 
far  as  the  mutations  are  "very  small,"  the  condition  after 
but  one  or  a  few  mutations  had  appeared  would  be  prac- 
tically indistinguishable  from  a  "transitional  condition"  to 
the  state  after  many  mutations  had  occurred.  Morgan 
(1917)  recently  insists  that  it  must  be  recognized  and  has 
always  been  urged  by  de  Vries  that  "mutations  may  be  very 
small  so  far  as  the  character  change  is  concerned." 

The  true  and  important  points  insisted  on  by  the  muta- 
tion theory  appear  to  be  these: — 

(1)  There  are  many  differences  ("variations")  between 
individuals  that  are  not  heritable.  Hence  by  selection  of 
such  diversities  no  evolutionary  change  is  produced. 


Nature  of  Inherited  Variations  £13 

(2)  Actual  hereditary  changes  in  characters  occur  rather 
rarely.     This  is  apparently  what  is  meant  by  the  statement 
that  they  appear  suddenly  ("sprungweise")  ;  for  a  time  they 
do  not  exist;  then  they  do.      (But  the  changes  thus  suddenly 
occurring  may  be  so  minute  as  to  be  hardly  detectible  until 
later  changes  in  the  same  direction  have  accentuated  them.) 

(3)  Heritable  changes  may  and  often  do  occur  in  large 
steps,  so  far  as  their  effect  on  the  developed  characters  of 
adults  is  concerned.      (This  is  an  important  fact;  equally 
important  is  the  fact  that  heritable  changes  may  be,  and 
often  are,  very  minute.) 

(4)  Appearances  indicate  that  the  changes  are  analogous 
to  (or  actually  are)  chemical  changes.     When  one  chemical 
compound  changes  into  another,  there  is,  it  is  held,  no  transi- 
tional condition  between  the  two,  and  the  same  is  believed 
to  be  true  for  hereditary  variations.     This   conception   of 
the  nature  of  hereditary  variations  accounts  for  the  fact  that 
they  often  show  in  the  adult  as  changes  of  large  extent ;  and 
at  the  same  time  it  fits  equally  well  the  minutely  graded 
hereditary  changes  that  likewise  occur.     For   there  is  no 
change  so  minute  that  it  may  not  be  chemical  in  its  nature. 
In   the   immense   organic   molecule,   with   its   thousands    of 
atoms,  a  shift  of  a  single  radical  or  single  atom  from  one 
position  to  another  is  a  chemical  change,  though  it  may  make 
a  difference  so  slight  as  to  be  almost  beyond  detection  by 
the  most  refined  means. 

Whether  a  doctrine  embodying  these  ideas  differs  from 
that  set  forth  by  Darwin  to  such  an  extent  as  to  deserve  the 
name  of  a  new  theory  may  be  doubted.  This  will  be  a  mat- 
ter of  individual  opinion. 

The  doctrine  that  hereditary  changes  must  occur  by  large 
steps  evidently  cannot  be  held.  But  what  bearing  on  the 
method  of  progressive  evolution  has  the  fact  that  they  often 
do  occur  by  such  steps, — not  by  a  series  of  gradual  altera- 


214          Life  and  Death,  Heredity  and  Evolution 

tions?  In  the  eye  of  Drosophila  variation  may  occur  from 
red  to  white  directly,  without  transitional  stages ;  or  from 
any  grade  to  any  other;  the  continuous  scale  of  colors  we 
have  mentioned  is  obtained  only  by  arranging  the  steps  in 
order.  Some  maintain  therefore  that  evolution  has  occurred 
by  such  large  steps,  not  by  gradations.  This  conception 
has  evidently  lost  the  compelling  force  it  seemed  to  have  be- 
fore hereditary  variations  in  minute  grades  were  detected. 
The  very  facts  in  such  an  organism  as  Drosophila  show  that 
there  is  nothing  to  prevent  a  passage  from  one  extreme  to 
another  by  minute  changes,  such  as  are  held  to  occur  by 
palaeontologists  and  selectionists.  Further,  in  such  cases 
as  the  eye-color  of  Drosophila  we  are  dealing  with  charac- 
ters that  are  already  highly  developed,  and  the  changes  we 
observe  are  mainly  retrogressive.  We  know,  for  example, 
that  the  red  eye  color  of  Drosophila  is  formed  by  the  co- 
operation of  many  separate  parts  of  diverse  chromosomes ; 
it  is  a  highly  complex  product  of  evolution.  Now,  we  find 
that  one  or  another  of  these  parts  may  suddenly  cease  to 
perform  its  function,  so  that  the  red  color  is  not  completely 
formed ;  there  is  a  sudden  change  in  it ;  or  it  may  disappear 
completely.  But  it  may  be  doubted  whether  this  implies 
that  in  the  original  production  of  this  complex  character, 
with  its  numerous  underlying  functional  parts,  there  was 
the  same  change  by  sudden  large  steps.  Is. there  any  rea- 
son to  suppose  for  example  that  at  one  time  there  was  a 
complete  eye  save  for  the  absence  of  the  red  color ;  and  that 
this  suddenly  appeared?  Our  knowledge  that  this  red  color 
is  made  by  the  cooperation  of  many  diverse  parts  makes 
such  a  notion  almost  inconceivable.  Destructive  changes  in 
a  fully  formed  character,  such  as  we  see  in  the  large  major- 
ity of  cases  of  mutation,  could  hardly  be  expected  to  throw 
light  on  how  that  character  was  built  up.  The  observed 


Nature  of  Inherited  Variation*  215 

facts  leave  readily  open  the  possibility  of  the  building  up  of 
a  character  by  minute  graded  changes. 

In  essentials,  therefore,  the  study  of  mutations,  when 
carried  so  far  as  in  Drosophila,  is  not  in  disagreement  with 
our  observations  of  gradual  variation  in  the  Protozoa  nor 
with  the  conclusions  of  palaeontologists  as  to  the  gradual  de- 
velopment of  the  characteristics  of  organisms  in  past  ages. 

These  conclusions  of  the  palaeonologists  are  well  stated 
in  the  recent  work  of  Osborn  (1917).  He  sets  forth  that  in 
following  given  stocks  from  earlier  to  later  ages,  characters 
arise  from  minutest  beginnings  and  pass  by  continuous 
gradations  to  a  highly  developed  condition.  This  seems 
in  agreement  with  the  experimental  results  on  both  higher 
and  lower  organisms,  as  I  have  tried  to  set  them  forth.  The 
palasontogolical  evidence,  he  holds  further,  indicates  that  the 
hereditary  changes  as  one  passes  from  age  to  age  do  not  oc- 
cur in  random  directions,  but  follow  a  definite  course,  which 
might  seem  to  have  been  predetermined  in  the  constitution 
of  the  organisms,  or  otherwise.  In  the  experimental  work 
on  the  lower  organisms  little  that  indicates  this  has  thus  far 
been  observed.  By  selection  we  can  move  in  more  than  one 
direction ;  though  it  is  also  true,  of  course,  that  the  varia- 
tions possible  are  limited  by  the  constitution  of  the  organ- 
ism. The  experimental  work  has  hardly  gone  far  enough 
to  offer  important  evidence  on  this  problem. 

There  is  one  other  point  in  the  work  on  higher  organisms 
that  we  may  briefly  consider.  This  is  the  point  made  by 
Bateson  (1914)  in  his  Presidential  Address  before  the  Brit- 
ish Association,  and  further  developed  in  a  recent  paper  by 
Davenport  (1916).  It  is  the  paradoxical  proposition  that 
since  practically  all  observed  variations  are  cases  of  loss 
and  disintegration,  we  are  driven  to  suppose  that  evolution 
has  occurred  by  loss  and  disintegration.  Davenport  com- 


216         Life  and  Death,  Heredity  and  Evolution 

bines  this  idea  with  the  theory  that  these  disintegrating 
variations  follow  a  definite  course,  predetermined  in  large 
measure  by  the  constitution  of  the  disintegrating  material. 

There  are  two  points  that  need  consideration  in  dealing 
with  this  theory.  The  first  is  one  of  observational  fact ;  al- 
though it  is  true  that  many  mutations  appear  to  be  cases  of 
loss  and  disintegration,  yet  there  is  no  indication  that  this 
is  the  case  in  such  results  of  selection  as  have  been  described 
in  the  Protozoa ;  heritable  variations  are  not  limited  to  any 
particular  direction. 

But  secondly,  it  appears  to  me  that  this  conclusion  that 
evolution  is  by  disintegration  and  loss  is  based  on  an  error 
in  logic,  which,  being  detected,  puts  it  out  of  consideration. 
As  we  examine  the  series  of  organisms,  from  amoeba  to  man ; 
or  as  we  examine  the  palaeontological  series,  we  find  a  grada- 
tion from  those  showing  little  visible  differentiation  to  those 
showing  great  visible  differentiation;  the  problem  of  evolu- 
tion is  as  to  how  the  passage  was  made  from  those  visibly 
little  differentiated  to  those  visibly  highly  differentiated. 

But  now,  according  to  the  doctrine  we  are  considering, 
when  we  come  to  examine  the  actual  changes  in  hereditary 
characters,  we  detect  on  the  whole  only  the  disintegration 
of  organisms  already  visibly  differentiated ;  only  a  change 
from  greater  visible  differentiation  to  less. 

The  doctrine  then  proceeds  to  draw  from  this  fact  the 
absurd  conclusion  that  the  visibly  more  differentiated  must 
have  arisen  from  the  visibly  less  differentiated  by  decrease 
in  the  differentiation  of  the  latter ! 

So  preposterous  a  conclusion  can  be  drawn  only  from  the 
fact  that  while  in  our  premises  we  are  talking  of  visible  in- 
crease and  decrease  of  differentiation,  in  the  conclusion  as 
ordinarily  drawn  the  ground  is  shifted  to  mean  something 
entirely  different, — an  inner,  invisible,  purely  theoretical 


Process  of  Evolution  217 

kind  of  disintegration  and  differentiation.  If  we  recall  that 
we  were  dealing  in  the  premises  with  visible  increase  and  de- 
crease of  differentiation,  and  that  therefore  in  the  conclusion 
we  must  deal  with  the  same,  the  absurdity  of  the  conclusion 
becomes  manifest.  All  that  we  can  legitimately  conclude, 
if  we  accept  the  premise  that  the  observed  changes  in  hered- 
itary characters  are  cases  of  loss  and  disintegration,  is  that 
we  have  not  seen  the  process  of  evolution  occurring.  But 
we  are  not  compelled  to  accept  that  premise.  In  the  lower 
organisms  at  least  it  cannot  be  asserted  that  all  changes  of 
hereditary  characters  are  cases  of  loss  and  disintegration. 

General  View  of  the  Processes  and  Problems  of  Development, 

Mating  and  Evolution,  in  the  Light  of  What  We 

Find  in  Lower  Organisms 

Let  us  now  attempt  an  outline  of  what  our  examination  of 
the  processes  of  mating  and  development  have  shown  us  in 
the  lower  organisms,  in  so  far  as  it  agrees  with  what  we  find 
also  in  the  higher  organisms. 

We  saw  at  the  beginning  that  each  species  of  organism, 
so  far  as  studied,  is  differentiated  into  many  slightly  diverse 
stocks,  each  diversity  hereditary.  This  we  found  to  be  true 
both  in  Protozoa  and  in  higher  organisms.  We  saw,  too, 
that  in  simple  reproduction  from  a  single  parent,  by  division 
of  a  cell  or  of  an  individual,  there  is  a  high  degree  of  con- 
stancy in  the  hereditary  characters  of  these  slightly  differ- 
ing stocks.  The  constancy  is  so  great  that  for  a  long  time 
the  search  for  hereditary  changes  was  at  a  standstill;  the 
stocks  seemed  permanent. 

But  with  intensified  study,  it  was  found  that  in  the 
Protozoa  changes  in  the  hereditary  characters  do  occur 
even  in  reproduction  by  simple  division.  Such  an  organism 
as  Difflugia  gradually  differentiates,  even  without  mating, 


218          Life  and  Death,  Heredity  and  Evolution 

into  slightly  differing  stocks,  similar  to  those  found  in  na- 
ture. The  changes  are  either  continuous,  or  by  steps  so 
slight  that  single  ones  are  hardly  detectible. 

In  the  higher  organisms,  where  the  matter  is  greatly  com- 
plicated by  the  fact  that  most  reproduction  is  from  two 
parents,  it  first  appeared  that  gradual  or  minute  hereditary 
changes  did  not  occur.  But  as  we  have  tried  to  bring  out 
above,  with  more  thorough  study  it  has  come  to  light  that 
such  changes  do  occur,  as  well  as  do  hereditary  alterations, 
which,  when  they  reach  the  adult  condition,  form  a  sudden 
marked  change.  In  this  way  all  contrast  in  principle  be- 
tween what  we  find  in  the  lower  organisms  and  what  we  find 
in  the  higher  ones  disappears.  In  both  the  process  of  evo- 
lution by  minute  gradations  is  visible. 

Then  we  proceeded  to  examine  how  far  in  lower  organ- 
isms diversity  of  external  conditions  brings  about  hereditary 
changes.  Here  again  the  first  examination,  even  though 
long  continued,  seemed  to  show  constancy;  diversity  of  ex- 
ternal conditions  appeared  to  have  no  permanent  effect  on 
the  stocks.  But  again,  intensified  study  reveals  that  the 
hereditary  characters  gradually  do  become  changed  by  di- 
versities of  external  conditions.  Through  such  diversities, 
continuing  for  great  numbers  of  generations,  single  stocks, 
uniform  in  their  hereditary  characters,  gradually  differen- 
tiate into  many  with  faintly  differing  hereditary  features. 
Again  the  process  is  gradual,  or  by  steps  so  small  that  sin- 
gle ones  are  imperceptible.  In  higher  organisms  the  state 
of  knowledge  on  this  point  appears  less  satisfactory.  But 
the  evidence  so  far  as  it  goes  indicates  that  the  processes 
here  are  in  agreement  with  those  in  lower  organisms.  Ap- 
parently diversities  in  external  agents  may,  under  condi- 
tions which  seem  rather  rarely  met,  so  modify  the  germ  cells 
that  they  produce  progeny  with  changed  hereditary  char- 


Process  of  Evolution  219 

acters.  On  the  other  hand  there  is  little  indication  that 
when  an  agent  produces  a  direct  effect  on  a  part  of  the  body, 
this  so  changes  the  germ  cells  that  in  later  generations  they 
produce  bodies  with  the  same  alterations. 

All  together  we  find  then,  that  even  independently  of  any 
mating  processes,  diversity  of  stocks  is  being  produced,  but 
most  slowly  and  gradually.  ^ 

Next  we  turned  to  a  study  of  reproduction  from  two  par- 
ents, and  its  relation  to  these  general  questions.  We  found 
that  while  in  many  organisms  (particularly  the  higher 
ones),  the  two  individuals  or  cells  that  mate  are  unlike,  be- 
longing to  separate  sexes,  this  seems  not  to  be  universal. 
Mating  is  apparently  often  between  like  parts.  This  ap- 
pears clearest  in  the  final  act  of  mating,  the  conjugation  of 
the  chromosomes,  for  in  these  there  is  no  indication  of  a 
sex  difference.  But  it  seems  to  be  true  also  in  many  cases 
for  the  germ  cells  and  for  the  individuals  that  mate.  It  is 
certainly  not  clear  that  sex  diversity  is  a  general  and  fun- 
damental requisite  for  mating ;  rather  does  the  contrary  ap.- 
pear  true. 

Yet  where  sex  diversity  does  occur,  as  in  the  higher  or- 
ganisms, it  is  manifest  in  the  most  fundamental  features  of 
the  organism.  Every  cell  of  the  male,  in  many  organisms, 
differs  from  every  cell  of  the  female,  and  precisely  in  the 
most  fundamental  features  of  the  cell ;  in  the  nuclei ;  and  in 
the  essential  chemical  operations  in  which  the  nuclei  are  in- 
volved. But  this  seems  to  be  a  condition  derived  from  the 
simpler  state  where  no  such  diversity  exists,  but  in  which 
mating  nevertheless  occurs. 

In  search  for  what  is  fundamental  in  mating  and  its  re- 
sults, we  came  upon  theories  that  mating  produces  re- 
juvenescence; that  mating  is  a  necessity  for  continued  ex- 
istence and  multiplication;  that  without  it  vitality  is  lost; 


220          Life  and  Death,  Heredity  and  Evolution 

and  that  it  must  take  place  in  order  that  the  lost  vitality 
shall  be  restored.  But  when  we  examine  the  evidence  on 
this  in  lower  organisms,  we  find  a  whole  series  of  facts  that 
will  not  range  themselves  under  this  doctrine,  along  with  some 
that  will.  After  mating,  some  organisms  are  less  vigorous 
than  before ;  some  little  altered ;  some  more  vigorous.  The 
latter  may  be  held  to  show  rejuvenescence.  But  the  gen- 
eral result  is  to  produce  many  diverse  stocks  with  new  sets 
of  hereditary  characters.  Some  of  these  new  combinations 
show  greater  vigor,  others  less ;  and  they  differ  in  many 
other  ways.  Such  diverse  stocks  resulting  from  mating 
show  similarities  among  themselves,  resulting  from  their  de- 
pendence on  the  union  of  two  parents ;  that  is,  they  show 
biparental  inheritance.  But  the  stocks  produced  by  a  given 
mating  differ,  too,  in  their  hereditary  characters. 

These  relations,  as  yet  little  known  in  the  lower  organisms, 
receive  illustration  on  a  vast  and  conspicuous  scale  in  the 
higher  organisms,  where  they  are  known  as  Mendelian  in- 
heritance. The  offspring  of  a  given  pair  are  more  alike 
than  are  the  offspring  of  diverse  pairs.  Nevertheless,  the 
offspring  of  a  single  pair  show  combinations  of  hereditary 
characters  diverse  from  each  other  and  from  their  parents. 

Examining  the  minute  processes  that  occur  in  reproduc- 
tion from  two  parents,  we  find  that  there  is  a  visible  forma- 
tion of  new  combinations  of  the  chemicals  on  which  develop- 
ment and  function  depend;  new  combinations,  of  the  pri- 
mary hereditary  characters.  These  chemicals  are  in  vis- 
ible packets,  and  the  method  of  forming  new  combinations 
of  them  can  be  seen  under  the  microscope.  Even  when,  as 
often  happens,  the  mating  is  between  parts  of  the  same 
nucleus  of  the  same  cell,  the  processes  are  such  as  to  bring 
about  new  combinations  of  the  primary  hereditary  sub- 
stances ;  organisms  with  new  combinations  of  hereditary 


Process  of  Evolution  221 

characters  are  produced.  These  recombinations  occur  in 
the  same  general  way  in  the  lower  and  the  higher  organisms. 

This  formation  of  new  combinations  of  the  primary  hered- 
itary substances  is  then  the  general  feature  of  mating.  It 
is  this  of  which  we  were  in  search  when  we  asked:  Is  there 
any  general  result  of  mating,  comparable  with  the  produc- 
tion of  energy  as  the  general  result  of  the  taking  of  food? 
Mating  is  a  process  of  forming  new  combinations  of  the 
primary  hereditary  materials. 

As  a  result  of  these  new  combinations,  the  organisms  pro- 
duced are  very  diverse  in  their  hereditary  characters.  In 
the  infusorian  some  are  weak  and  unenduring;  the  things 
combined  do  not  work  to'gether  harmoniously ;  they  die  out. 
Others  are  strong  and  vigorous ;  they  persist  and  multiply. 
In  other  organisms  similar  differences  appear,  along  with  di- 
versities in  respect  to  all  possible  hereditary  characteris- 
tics. Thus  mating  steadily  changes  the  face  of  organic  na- 
ture, continuously  producing  new  combinations,  some  of 
which  are  extinguished,  while  others  flourish. 

This  process  is  greatly  assisted  in  the  lower  organisms  by 
the  fact  that  after  a  set  of  new  combinations  is  produced  by 
mating,  each  combination  is  multiplied  greatly  by  vegeta- 
tive reproduction,  which  does  not  make  a  change  in  the 
grouping.  Thus  each  combination  is  given  an  opportunity 
to  meet  many  diverse  external  conditions,  with  some  of  which 
it  may  work  in  harmony;  further,  the  number  of  possible 
diverse  combinations  which  may  result  from  the  next  period 
of  mating  is  greatly  increased. 

This  continued  formation  of  new  combinations  is  the 
great  corrective  of  the  uniformity  which  would  result  from 
more  rigid  laws  of  heredity.  In  general,  no  one  can  predict 
what  combinations  will  actually  result  from  a  given  mating, 
for  the  number  possible  is  much  greater  than  the  number 


222         Life  and  Death,  Heredity  and  Evolution 

that  can  be  realized.  In  other  words,  no  one  can  predict 
with  certainty  the  characteristics  of  the  offspring  to  be 
produced  by  a  given  pair.  Parents  in  which  certain  char- 
acteristics are  developed  in  but  a  mediocre  degree  may  pro- 
duce by  their  union  offspring  in  which  these  characteristics 
are  developed  in  a  high  degree,  as  we  saw  in  Paramecium  that 
parents  of  low  vigor  may  produce  a  few  offspring  of  high 
vigor.  This  is  as  true  for  the  qualities  which  in  human  in- 
dividuals we  tend  to  class  as  good  or  bad,  as  for  vigor  in 
Paramecium.  From  mediocre  parents  may  arise,  by  the 
formation  of  new  combinations  of  the  hereditary  material, 
offspring  that  are  distinguished  for  good  or  for  ill. 

This  is  the  fact  that  undermines  all  exclusively  aristo- 
cratic theories  of  breeding  and  inheritance  in  such  an  organ- 
ism as  man;  this  is  the  possibility  that  must  underlie  any 
democratic  theory  of  society  and  of  progress.  Possibly 
from  the  great  mass  of  mediocre  humanity  there  may  arise 
by  new  combinations  in  every  generation  so  great  a  number 
of  distinguished  men  as  to  make  the  contribution  of  offspring 
from  the  relatively  few  distinguished  individuals  unimpor- 
tant. It  is  not  certain  that  the  relative  infertility  of  the 
intellectual  classes  decreases  the  existing  proportion  of  in- 
tellectual men.  It  may  be  that  there  continually  arises  from 
the  great  average  mass  of  mankind  a  proportion  of  dis- 
tinguished men  that  remains  relatively  constant,  even  though 
these  distinguished  men  may  not  reproduce  at  all. 

To  return  to  our  general  relations,  this  process  of  re- 
combination does  not  evidently,  of  itself,  result  in  the  pro- 
duction of  any  new  characteristics.  We  cannot  say  posi- 
tively that  it  does  not,  but  if  the  chemicals  on  which  devel- 
opment depends — the  primary  hereditary  characters — were 
permanent,  unchangeable  things,  then  recombination  could 
produce  merely  kaleidoscopic  regrouping  of  these;  and  the 


Process  of  Evolution  823 

number  of  possible  diverse  combinations  that  could  appear 
would  be  definitely  limited.  All  change  would  be  a  regroup- 
ing of  what  already  exists. 

Therefore  it  becomes  most  important  to  examine  what 
happens  when  there  is  no  such  regrouping,  in  heredity  from 
a  single  parent.  Here  we  discover,  as  we  have  already  set 
forth,  that  actual  changes  in  the  hereditary  materials  are 
occurring,  independently  of  recombinations.  New  charac- 
teristics appear  that  are  heritable. 

Are  such  new  characters  bound  up  with  the  other  primary 
hereditary  characters,  and  subject  to  the  same  processes  of 
recombination  at  mating?  In  Drosophila  this  question  is 
answered  clearly  in  the  affirmative ;  the  new  characters  that 
appear  recombine  at  mating  as  do  the  old  ones.  There  is  lit- 
tle reason  to  doubt  that  they  do  so  in  all  organisms. 

Thus  a  new  character  arising  in  a  particular  individual 
having  a  given  combination  of  characters,  is  transferred  by 
mating  to  other  individuals  with  other  combinations.  With 
some  it  may  work  harmoniously ;  with  others,  not.  Further, 
several  new  characters  arising  in  diverse  individuals  may  by 
mating  become  transferred  to  a  single  one,  where  they  may 
form  a  combination  working  more  harmoniously  together 
and  with  the  environment  than  any  that  has  before  existed. 
Thus  all  sorts  of  combinations  arise,  of  new  and  old  charac- 
ters, such  as  could  not  possibly  occur  without  mating. 
Some  are  more  vigorous  and  harmonious ;  some  less ;  some 
fit  one  set  of  outward  conditions ;  some  "another.  Mating 
thus  contributes  enormously  to  the  differentiation  of  organ- 
isms into  hereditarily  diverse  stocks.  Its  general  result  is  to 
give  all  sorts  of  combinations  of  characters,  new  or  old; 
some  persisting,  others  not.  Added  to  the  slow  production 
of  new  characters,  it  greatly  hastens  the  changes  in  organic 
nature  that  we  call  evolution. 


WORKS    REFERRED    TO    IN    THE    TEXT. 

(This  is  not  a  general  bibliography  of  the  subject,  but  only  a  list  of 
the  works  to  which  reference  is  made.  Where  there  is  general  reference 
to  a  large  series  of  investigations  by  a  particular  author,  as  a  rule  only 
one  reference  is  given,  to  some  important  contribution;  from  which  the 
reader  can  follow  up  the  subject  if  he  desires.  This  is  done  particularly 
when  the  subject  lies  to  one  side  of  our  main  field.) 

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Calkins,  O.  N.,  1902.  Studies  on  the  life-history  of  Protozoa.  I.  The 
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INDEX 


Abnormalities,  150,  159 
Acclimatization,  97 
Ackert,  63 

Acquired  characters,  44 
Agar,  64 
Alcohol,  203 
Amoeba,  15,  39 
Amphibia,  200 
Anabolism,  115 
Anoplophrya,  180,  192 
Arcella,  56,  77,  S2 
Arsenic,  101 
Assortative  mating,  191 
Aster,  117 
Autogamy,  134,  185 

Bacillus  coli,  88 

Bacillus  prodigiosus,  63,  88 

Bacteria,  15,  50,  58,  63,  86-96 

Balbiani,  24 

Barber,  63,  87 

Bateson,  68,  215 

Beans,  64 

Biparental    inheritance,    151,    152, 

168,   170-190 

Biparental  reproduction,  21,  219 
Blakeslee,    123 
Blepharisma,  192 
Bott,  183 
Bridges,  208,  210 
Brown-S6quard,  203 
Biitschli,  24 
Butterflies,  202 

Ca?nomorpha,  16 

Calkins,  24,  29,  32,  120,  141,  151 

Calkins  and  Cull,  181 

Calkins  and  Gregory,  142 

Carchesium,  180 

Castle,   203,   207 

Catabolism,  116 

Centropyxis,  56,  77 

Centrosome,  116 

Chemical  agents,  88-96,  102,  203 


Chilodon,  128,  180 
Chlamydomonas,  155-157 
Chromidium,  83 
Chromosomes,    108,   115,   116,   138, 

171-190 

Climacostomum,  150 
Collin,  126,  180,  192 
Colored  bacteria,  88 
Colpoda,  165 

Constancy  of  stocks,  62,  65,  67,  86 
Corrosive  sublimate,  89 
Coulter,  126 

Conditions   of   conjugation,    164 
Conjugation,  22,  25,  36,  102,  106- 

169 

Continuity  of  life,  19,  21,  30,  35 
Corpse,  35 
Correlation,  73 
Crustacea,  64 
Cryptochilum,  165 
Cull,  120 

Dallinger,  98 

Dallingeria,  98 

Davenport,  68,  215 

Death,  19,  28,  39 

Democracy,  222 

Depression,  148 

Dexter,  208 

Didinium,    16,    32,    133,    142,    150, 

177,   195 

Difflugia,  40,  45,  50-55,  68,  70-76 
Diplodinium,  17 
Dobell,  93,  95,  96,  106,  153 
Doflein,  119 
Draba,  59 
Drosophilia,  205,  207,  208,  223 

Endomixis,  31 

Enriques,  29,  33,  128,  165,  180 

Environmental    modifications,    85- 

103,  198,  218 
Epistylis,  121 
Erdmann,  166 


231 


232 


Index 


Euplotes,  146 

Evolution,  38,  65-73,  83,  213,  223 

Ewing,  64 

Exchange  of  nuclei,  25,  36,  110 

Exconjugant,  112 

Eye  color,  208-211 

Female,  115,  116,  133 

Fermor,   32 

Fertilization,    36,    106 

Fischer,  202 

Fission  rate,  24,  81,  141-147,  154, 

158,  161 
Flagellate,  134 
Folliculina,  16 
Fowl,  203 

Geddes  and  Thompson,  115 
Genotype,  65,  86 
Glaucoma,  29,  33 
Gradations,   76,  209,   213-218 
Gregarinidae,  175 
Guinea  pig,  203 

Hance,  58 

Hartmann,  137 

Hegner,  56,   77,  82 

Heredity,    37-49,     67-84,    152-157, 

170-190 
Heritable    variations,    76,    81,    84, 

212 

Herpetomonas,  183 
Hertwig,  R.,  24,  142,  144-148 
Hutchison,  57 
Hydra,  64 
Hypotricha,  42 

Immortality  of  unicellular  organ- 
isms, 20,  30,  35 

Infusoria,  16,  17,  24 

Inheritance,  37-49,  67-84,  152-157, 
170-190 

Inheritance  of  acquired  characters, 
44,  48,  85-105,  199,  218 

Isolation  of  bacterium,  87 

Johannsen,  64,  65 

Jollos,  33,  34,  57,  63,  95,  101 

Jordan,  59,  66 

Kammerer,  200,  202 
Kinetic,  116 


Kinetonucleus,  96,  97,  116 
Kinetoplasm,  116 

Lacrymaria,  16 
Lashley,  64,  124 
Leucophrys,  28,  146,  150 
Lotsy,  59 

MacDowell,  207,  208 
Macronucleus,  25,  31,  35,  44,  103, 

110 

Male,  115,  116,  133 
Massini,  92 
Mast,  142,  195 
Maturation,  172-186 
Maupas,  24,  28,  30,  64,  114,  121,  122, 

131,  132,  142,  145,  146,  147,  150 
Mendelian  inheritance,  65,  187-190 
Metcalf,  181 
Micronucleus,  25,  31,  44,  58,   103, 

110 

Middleton,  80,  100 
Migratory  nucleus,  110,  132 
Minchin,  126 
Modification,  103 
Modification  of  inherited  characters 

by  external  agents,  85-103,  198, 

218 

Modifying   factors,  207,  210 
Monas,  98 
Monocystis,  116 
Monstrosities,  150,  159 
Morgan,  205,  207,  208 
Mould,  123,  136 
Mucor,  123 
Muller,  208 
Mulsow,  175 

Multiple  allelomorphs,  209 
Mutation,  67,   68,   102,  204-211 
Mutilations,  inheritance  of,  45 
Myxobacteria,  93 
Myxococcus,  93 

Newman,  60 
Non-disjunction,  210 
Nucleo-plasmic  relation,  148 
Nucleus,  24,  25,  82 

CEnothera,  205 
Onychodromus,  28,  146 
Opalina,  181 
Opercularia,  128,  180 
Osborn,  215 
Oxytricha,  28 


Index 


233 


Palaeontology,  69,  215 
Paramecium,  22,  25,  29,  31,  34,  46, 

50,  56,  58,  63,  68,  97,  110,   119, 

130,  133,  142,  143,  154,  158,  165, 

181,  191 

Parthenogenesis,  64 
Pearl,  68,  203 
Pelomyxa,  183 
Plant  lice,  64 
Pneumonia  bacillus,  90 
Popoff,  180 
Potassium  bichromate,  effects  of, 

89 

Potato  beetle,  202 
Powers  and  Mitchell,  58 
Prandtl,  133,  177 
Primary  hereditary  characters,  170, 

185-187,  222 
Pringsheim,  95 
Protozoa,  15 
Prowazek,  183 
Pure  line,  65 
Purpose,  106 

Quehl,  93 

Radl,  13 

Rate  of  reproduction,  24,  81,  141- 

147,  154,  158,  161 
Reduction,  172,  175-186 
Reeves,  207 
Rejuvenescence,    23,    26,    36,    114, 

141-147,  150,  162,  220 
Reproduction,  rate  of,  24,  81,  141- 

147,  154,  158,  161 
Reserve  nucleus,  25 
Resistance,  57,  101 
Root,  56,  77 

Saltation,  205 

Schaudinn,  183 

Secondary    hereditary    characters, 

185 

Selection,  60-73,  81 
Senility,  30 

Sex,   109,  114-140,  168,  219 
Size  of  nuclei,  82 
Spirogyra,  135 
Spirostomuui,  150 


Standfuss,  202 
Stentor,  16 
Stephanosphaera,  126 
Stockard,  203 
Structure  of  Protozoa,  15 
Sturtevant,  207,  208 
Stylonychia,  16,  28,  32,  43,  81,  100, 
145,  150 

Temperature,    acclimatization    to, 

98-100 

Tetramitus,  98 
Toenniessen,  90 
Tower,  202 
Trichomastix,  134 
Trophic,  116 
Trophonucleus,    116 
Trophoplasm,  116 
Trypanosoma,  96,  183 
Twins,  60 

Unicellular  organisms,  15,  16 
Uniparental  inheritance,  38-84 
Uniparental  reproduction,  21 
Unit  factor,  208 

Uroleptus,  32,  142,  144,  149,  151, 
163 

Vacuoles,  contractile,  58 
Variation,  50,  61,  66,  67-72,  76,  193 
Vitality,  24,  141-147,  161,  196 
Vries,  de,  205,  212 
Vorticella  16,  120,  127 

Wallengren,  121 
Walton,  195 
Watters,  192 
Wolf,  63,  88,  93 
Woodruff,  29,  30,  33,  149,  167 
Woodruff  and  Erdmann,  30,  32,  34, 
103,  145 

Yeasts,  63 
Young,  33 

Zeleny  and  Mattoon,  207 
Zweibaum,  165 
Zygorhynchus,  136 
Zygospore,  123 


A     000670635 


